Remove usused image core/misc/loading.gif

The fix seems to be straightforward. Tested the MR by manually using the grep command.

grep -r 'loading.gif' core

Before applying the patch.

After applying patch

I checked where this was added (by running git log -S loading.gif) and it was in #610234: Overlay implementation → so was probably only ever used in the overlay module.

However the test failure is real, needs more than just removing that line from the test.

https://git.drupalcode.org/issue/drupal-3448412/-/jobs/1640473

Addressed #7, please review

Tests appear green and no searched core for core/misc/loading.gif but found nothing

Any concern that a contrib could be using this?

There seems like a non-zero chance that contrib could be using this. We can't deprecate as such because there's nothing to deprecate, but could add a change record just in case, and only remove it from 11.0 (I think it's fine during beta since it's dead-not-even-code).

Moving to needs work for the change record.