Stop generating invalid buffer loot table

[TeamDman]

Summary

• Stop generating a loot table for the WIP buffer block, which has no production item registration.
• Remove the stale generated loot table that references the nonexistent sfm:buffer item.

The previous datagen path emitted sfm:blocks/buffer with an sfm:buffer item entry even though SFMItems.BUFFER is only registered in the IDE. This produced an error during every resource reload.

Fixes #583

Related: #568, #530

Validation

• sfm-propagate-changes.exe run data --branch 1.19.2
• git diff --check