[ios] Make `-[EXSession deleteSessionFromKeychainWithError]` idempotent

This matches Android's behavior, where we can delete Home's session secret idempotently. This way we don't get errors if we cl

[ios] Make `-[EXSession deleteSessionFromKeychainWithError]` idempotent

This matches Android's behavior, where we can delete Home's session secret idempotently. This way we don't get errors if we clear the session token even after it's been cleared.

Tested by calling `LocalStorage.removeSessionAsync()` twice on Home and didn't get an async error.

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[android][ios][home] persist session info in one location on clients (#2521)

[android][ios][home][jest-expo] persist session info in one location on clients