This reverts commit 01ac144b09.
Looks like this broke the build on some systems, because some
package managers pack SDL2Config.cmake and others pack sdl2-config.cmake
in their SDL2 development packages (for some reason, SDL2 seems
to ship both in their source, and they appear to be incompatible).
Shipping our own FindSDL2.cmake might be unclean/ugly/whatever, but
at least it works (most of the time? at least it appears to work better
than not shipping it)