Remove --x-localimage experimental feature (#2670)
Remove the --x-localimage flag and all associated code that was never
shipped. This includes:
- CLI flag from build, push, predict, run, debug, and serve commands
- LocalImage field from BuildOptions and log contexts
- localImage parameter from dockerfile generators
- FillInWeightsManifestVolumes and WeightManifest (dead code)
- CogWeightsManifestLabelKey label constant
- Integration tests for the feature
