Removes ~517 lines of unreferenced code that has been superseded by the
Go-side FakeDNS path (mobile/tun/fakedns_proxy.go) since commit 80d78aa.

Kotlin side:
- Delete the legacy dns/ package (FakeDnsServer.kt, FakeDnsInterceptor.kt).
  The active FakeDNS path is Go-side through Mobile.startTunBridge(...).
- Delete service/BootReceiver.kt (15-line comment-only stub). The matching
  commented-out RECEIVE_BOOT_COMPLETED permission block and <receiver>
  entry are removed from AndroidManifest.xml. Auto-connect can be restored
  from git history when an explicit Settings UI lands.

Go side (mobile bridge surface shrink):
- mobile/mobile.go: remove GetVersion, GetListenAddress,
  IsTunBridgeRunning, GetDNSMapping, GetDNSMappingCount, GetTunVersion.
  None of these are called from Kotlin (verified by grep). The unused
  internal/version import is dropped. Preserved: StartClient, StopClient,
  IsRunning, StartTun, StopTun, StartTunBridge, StopTunBridge,
  GetTunBandwidth (the live one reading trackedUp/trackedDown that
  VpnManager.kt:277,288 calls).
- mobile/tun/tun_api.go: remove the dummy GetTunBandwidth (shadowed by
  the mobile.go version that Kotlin actually calls) and GetVersion
  (would collide with mobile.go's binding export; never reachable from
  Kotlin once mobile.go:GetTunVersion is gone).

Verification:
- go build ./mobile/tun/... exits 0
- go test ./mobile/tun/... (plan 006 tests) passes
- git diff --quiet -- go.mod go.sum (unchanged; AGENTS.md invariant held)
- 0 Android references to FakeDnsServer|FakeDnsInterceptor|BootReceiver
- 0 Kotlin Mobile.* callers reference any deleted symbol

Local top-level `go build ./mobile/...` and the gradle/AAR build are
gated by .github/workflows/android-ci.yml on push (pre-existing
syscall.Dup-undefined-on-Windows blocks the top-level Go build locally,
independent of this commit's edits).