Firestore: Remove obsolete special case from tests when verifying "missing index" error message in non-default DB

[dconeybe]

This PR removes a special case from the integration tests (#12189) that verifies the "missing index" error message returned from the server. Previously, a non-default database would result in a different, less descriptive error message; however, the backend was updated to use the same, descriptive error message for both default and non-default databases. The tests, therefore, should have to the special case removed to increase the coverage of the tests.

Googlers see b/316359394 for more information.

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Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.09% (75f5282) to 88.10% (5516dcc) by +0.01%.

  Filename	Base (75f5282)	Merge (5516dcc)	Diff
  ordered_code.cc	93.90%	94.39%	+0.49%
  task.cc	96.52%	94.78%	-1.74%
  write_stream.cc	91.55%	94.37%	+2.82%

Test Logs

• Base (75f5282): https://github.com/firebase/firebase-ios-sdk/actions/runs/7428588353
• Merge (5516dcc): https://github.com/firebase/firebase-ios-sdk/actions/runs/7452175778

1. https://storage.googleapis.com/firebase-sdk-metric-reports/coVmQzxpmk.html