refactor(proto): remove legacy scan field

[Phoenix500526]

Which issue does this PR close?

• Closes #N/A.

Rationale for this change

Since DataFusion doesn't typically guartantee wire format compatibility, I remove the backward compatiblity shim that introduced in PR #23189

FYI: #23189 (comment)

What changes are included in this PR?

DataFusion does not guarantee serialized plans across versions. Keeping partitioned_by_file_group therefore leaves a dead schema field and decoder path after output_partitioning became the source of truth. Reserve the old field number and name to prevent future reuse.

Are these changes tested?

Yes

Are there any user-facing changes?

Yes

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-proto v54.0.0 (current)
       Built [  60.258s] (current)
     Parsing datafusion-proto v54.0.0 (current)
      Parsed [   0.019s] (current)
    Building datafusion-proto v54.0.0 (baseline)
       Built [  58.978s] (baseline)
     Parsing datafusion-proto v54.0.0 (baseline)
      Parsed [   0.019s] (baseline)
    Checking datafusion-proto v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.254s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [ 120.800s] datafusion-proto
    Building datafusion-proto-models v54.0.0 (current)
       Built [  24.353s] (current)
     Parsing datafusion-proto-models v54.0.0 (current)
      Parsed [   0.124s] (current)
    Building datafusion-proto-models v54.0.0 (baseline)
       Built [  23.675s] (baseline)
     Parsing datafusion-proto-models v54.0.0 (baseline)
      Parsed [   0.127s] (baseline)
    Checking datafusion-proto-models v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   1.864s] 223 checks: 221 pass, 2 fail, 0 warn, 30 skip

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/inherent_method_missing.ron

Failed in:
  FileScanExecConf::partitioned_by_file_group, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/2fb7dbc2d6a1d5d763340c70b7f3e4a3f36d0d61/datafusion/proto-models/src/generated/prost.rs:1793
  FileScanExecConf::partitioned_by_file_group, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/2fb7dbc2d6a1d5d763340c70b7f3e4a3f36d0d61/datafusion/proto-models/src/generated/prost.rs:1793

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field partitioned_by_file_group of struct FileScanExecConf, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/2fb7dbc2d6a1d5d763340c70b7f3e4a3f36d0d61/datafusion/proto-models/src/generated/prost.rs:1818
  field partitioned_by_file_group of struct FileScanExecConf, previously in file /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/2fb7dbc2d6a1d5d763340c70b7f3e4a3f36d0d61/datafusion/proto-models/src/generated/prost.rs:1818

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [  51.412s] datafusion-proto-models