Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

The latter signal was likely used by mistake, e.g. https://github.com/dolphin-emu/dolphin/pull/8263.

qOverload is used to disambiguate pointers to overloaded functions, but
most of the usages in the code base were with non-overloaded functions.

In all instances, these variables should be signed rather than unsigned.