refactor: clean up legacy Swift 5.x version checks

[ncooke3]

Recommend hiding whitespace: https://github.com/firebase/firebase-ios-sdk/pull/16305/changes?w=1

Since the minimum supported Swift version across the SDK has been bumped to 5.9, #if swift(>=5.x) checks and their corresponding #else blocks evaluate to true by default and are no longer necessary. This commit removes the redundant directives.

#no-changelog

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[ncooke3]

/gemini review

[gemini-code-assist]

Code Review

This pull request removes obsolete Swift version check compiler directives (#if swift(...)) across several test files and utility sources, including InstallationsAPITests.swift, AsyncAwaitIntegrationTests.swift, DatabaseTests.swift, and HashCalculator.swift. This simplifies the codebase by assuming a modern Swift environment (Swift 5.5+). There are no review comments, and I have no feedback to provide.