Script Loader: Remove the unused compression test.
The compression test ran on admin screens until it stored its result in the can_compress_scripts option, but that result has had no effect since PHP compression was removed from load-scripts.php and load-styles.php in r43580, and the last code reading it was removed in r64485. The test also failed with a 400 error when admin_url() was filtered to add a query string, as translation plugins do, since it appended a second ? to the AJAX URL. Users without manage_options were sent the test on every admin page, since the AJAX handler rejected their requests and the option was never set. Rather than fixing the URL, the test is removed.
The compression_test() and wp_ajax_wp_compression_test() functions are deprecated as no-ops so that existing callers do not cause fatal errors, and the wp-compression-test AJAX action is no longer registered. On single sites, the option is already deleted on a database upgrade since it is in the list of unused options. On multisite it is a network option, so it is no longer primed and is now deleted by upgrade_network(), with the database version bumped for that step.
Developed in https://github.com/WordPress/wordpress-develop/pull/14412.
Follow-up to r10357, r43580, r64485.
Props azaozz, juzar, tofandel, szepeviktor, irozum, oglekler, peterwilsoncc, westonruter.
See #57548, #63017.
Fixes #58302, #65750, #65980.
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