[X86] Remove the DeprecatedMPX feature flag.

We deprecated mpx feature in 10.0. I left this feature flag
in case someone still had IR files containing the feature
in a target-feature attribute. At the time I think I thought it
would fail the test if the feature couldn't be found. Further
review suggests that at worst it prints a message to
stderr about ignoring the feature.

GitOrigin-RevId: ebe5f17f9c396f14742a7c747951b65eaada158e