Externals: Fix clashing dependencies

Our FindLibUSB.cmake was previously entirely unused unless SDL was being built from Externals, we now rely on it again. It will use PkgConfig if applicable or fall back to looking around on the system, and more importantly it will always create an imported target.

Currently, configuring from System>Bundled doesn't work, it instead produces cryptic errors. And configuring from Bundled>System wont produce errors, but wont use the system libraries either. This change produces a clear error in both cases.

OProfile is not used at all these days, most major distributions do not ship it anymore (Debian, Fedora, and Alpine to name the few I've checked) and following a discussion on Discord, nobody is apparently using it, most devs not even being aware of it. This removes an optional dependency from Dolphin.