Remove deprecated sync -[FIRInstanceID appInstanceID:] method

[maksymmalyhin]

The method must be removed and [FIRInstanceID appInstanceID:] , all usages must be replaced the the async alternative.

[charlotteliang]

If other SDKs like analytic stop using this, we should remove it. Thanks for filing this!

[maksymmalyhin]

All Firebase SDKs (except FCM so far) have been updated to use FIS directly, but the method may still be used in some 1P tests.

[charlotteliang]

I think it's okay to remove it. I couldn't find any 1P depending on it anymore.
Since it's a private API, is it considered a breaking change? ( I see we mark it firebase 7)

[paulb777]

We need to wait for Firebase 7 - because earlier Firebase 6 versions of other pods allow minor version updates of IID. If an old version of Firebase Analytics updates and this API is removed, the build will break.