Remove --ios_cpu flag

[keith]

This flag appears to be unused, it was set once but never read. Instead
users should prefer --ios_multi_cpus or --cpu=ios_XXX

[lberki]

/cc @allevato

[allevato]

This one would need some internal repo cleanup before we could accept it; we still have a few teams using --ios_cpu in some scripts and CI config files.

[aiuto]

@allevato Should we close this PR and assume you will do it after the internal cleanup?

[allevato]

Yes, this is cleanup that we'll most likely do after we've finished the transition over to platforms. (I don't see a need to rush it at this point; it would be better to tidy up all of that legacy stuff holistically.)

[keith]

I reopened this one since we actually hit some related issues here. If you're getting a transition from bazel itself it includes ios_cpu, but the starlark transitions that have been written so far don't actually include it, which in some cases causes issues. We can make the external transition provide this value to match with our expectations, but given that this flag has been deprecated for quite a while now, it seems like a good chance to remove it and reduce confusion around which flag should be used.

Can someone from google remove the internal uses of this flag?

Related: bazelbuild/rules_apple#1579

[aiuto]

@allevato, cc: @gregestren, @katre

Tony: I see fewer ios_cpu instances each month. Do you have a status update on internal movement.

[keith]

seems like this was mostly handled in ecf9ac9