Removes sourceTargets migration code.
Removes sourceTargets migration code.

Users have had awhile to migrate now, so there's no compelling need to
maintain support for the old sourceTargets in configs.

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MOS_MIGRATED_REVID=120232488

bazelbuild/tulsi,pinterest/tulsi,pinterest/tulsi,pinterest/tulsi,pinterest/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,pinterest/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,pinterest/tulsi,bazelbuild/tulsi