Remove unused headers

Remove the [[maybe_unused]] annotation from various parameters that are
unconditionally unused and comment out their names instead. This makes
it unambiguous that the variables are unused, while making the remaining
[[maybe_unused]] annotations more reliable indicators that those
variables are in fact used in some contexts.

These parameters are mostly in overridden functions where the override
doesn't need that particular variable.

Source/Core: Replace `find(x) != npos` with `contains(x)`

EGL is the de-facto GL context initialization API, including on X11
where it provides many additional features over GLX.

I’m planning on adding support for selecting the GPU (adapter in
Dolphin-speak) also to OpenGL, similarly to the Vulkan backend, and that
will require EGL, so let’s remove the legacy API first.

Accomplished using `run-clang-tidy` with `performance-move-const-arg,performance-unnecessary-value-param,modernize-pass-by-value`.

Changed arguments to const references, removed them where inappropriate (e.g. sink parameters). Same with std::move.

Manually reviewed each change to make sure that it makes sense, and do something more appropriate if possible.

DX, OGL: fix assert

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...)