fix(auth): don't remove items from keychain if it already has items

[harsh62]

Issue #

#4098

Description

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

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

[codecov]

Codecov Report

❌ Patch coverage is 31.25000% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.63%. Comparing base (76c8ddc) to head (dc16882).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/Core/AWSPluginsCore/Keychain/KeychainStore.swift	0.00%	15 Missing ⚠️
...dentialStorage/AWSCognitoAuthCredentialStore.swift	58.82%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4105      +/-   ##
==========================================
- Coverage   66.87%   66.63%   -0.25%     
==========================================
  Files        1129     1129              
  Lines       42585    42616      +31     
==========================================
- Hits        28478    28396      -82     
- Misses      14107    14220     +113     

Flag	Coverage Δ
API_plugin_unit_test	68.38% <ø> (ø)
AWSPluginsCore	68.07% <0.00%> (-0.47%)	⬇️
Amplify	46.72% <ø> (ø)
Analytics_plugin_unit_test	84.84% <ø> (ø)
Auth_plugin_unit_test	72.36% <58.82%> (-0.20%)	⬇️
DataStore_plugin_unit_test	81.78% <ø> (-0.93%)	⬇️
Geo_plugin_unit_test	73.39% <ø> (ø)
Logging_plugin_unit_test	64.86% <ø> (ø)
Predictions_plugin_unit_test	33.89% <ø> (ø)
PushNotifications_plugin_unit_test	87.41% <ø> (ø)
Storage_plugin_unit_test	77.68% <ø> (ø)
unit_tests	66.63% <31.25%> (-0.25%)	⬇️

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