Remove deprecated credential methods from credential providers

[mullermp]

This change removes deprecated methods. We will merge this in as core version 3.75.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[cjyclaire]

Looks good, likely would need to rebase again before final merging

[edma2]

Given that this change is backwards incompatible, wasn't it supposed to be released with a major version bump? According to the documentation:

You can safely express a dependency on a major version and expect all minor and patch versions to be backwards compatible.

[mullermp]

@edma2 For the change prior to this, the documentation indicated that they were to be removed in version 2 but never were. Even with that miss, it should have been removed in version 3 (prior to my time).

    # @deprecated Deprecated in 2.1.0. This method is subject to errors
    #   from a race condition when called against refreshable credential
    #   objects. Will be removed in 2.2.0.

We did so because they were buggy to begin with, and that they had been marked as deprecated for such a long time. This was communicated not just in the comments but as well as log statements. I'm sorry for this inconvenience, but you can fix it just by calling the credentials method on the provider.