Revert "Switching from UNAUTHENTICATED to UNAVAILABLE for auth metadata failure"

[jtattermusch]

Reverts #13322

The PR changes call statuses without updating tests for all languages. There are breakages at least in C#, Ruby and PHP (on all platforms).

E.g. in c#

1) Failed : Grpc.IntegrationTesting.MetadataCredentialsTest.MetadataCredentials_InterceptorThrows
  Expected: Unauthenticated
  But was:  Unavailable
at Grpc.IntegrationTesting.MetadataCredentialsTest.MetadataCredentials_InterceptorThrows()

[grpc-testing]

****************************************************************

libgrpc.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]


****************************************************************

libgrpc++.so

     VM SIZE        FILE SIZE
 ++++++++++++++  ++++++++++++++

  [ = ]       0        0  [ = ]

[grpc-testing]

[trickle] No significant performance differences

[grpc-testing]

[microbenchmarks] No significant performance differences

[jtattermusch]

Merging early to fix the breakage.

[jtattermusch]

@jboeuf, feel free to recreate the PR and ask others to create PRs against your branch to update tests for wrapped languages before the new PR gets merged (but the updates should be fairly trivial - e.g. for C# it's a change of an assertion)

[jboeuf]

@jtattermusch sorry for the breakage. Would you (or someone else from the gRPC team) mind recreating the PR and fix the different languages? I'm really swamped now. Thanks much.
@ejona86 FYI.