Tests: Remove unused properties and helper methods from test classes

[Soean]

Removes dead code from the PHPUnit test suite: six private properties that are never read, and nine helper methods in Tests_HTTPS_Detection that lost their callers when changeset 56664 removed the tests hooking them onto pre_http_request.

No test behaviour changes.

Follow-up to r32648, r43584, r54799, r56664, r61700.

Trac ticket: https://core.trac.wordpress.org/ticket/65819

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Locating the unused properties and methods and verifying they have no remaining callers; the changes were reviewed and committed by me.

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