sdl2: remove SDL2-static target from sdl2-config.cmake

/usr/lib/x86_64-linux-gnu/cmake/SDL2/sdl2-config.cmake provides targets to use SDL2 with cmake. One of the target, SDL2::SDL2-static, requires the file /usr/lib/x86_64-linux-gnu/libSDL2.a in order to work correctly but this file is removed with the command find "%{install-root}" -name "lib*.a" -exec rm {} ";".

Without the libSDL2.a file, flycast failed to build with the following error (https://flathub.org/builds/#/builders/35/builds/8609):

ninja: error: '/usr/lib/x86_64-linux-gnu/libSDL2.a', needed by 'build/flycast', missing and no known rule to make it

An alternative solution could be to remove the part related to the static library (https://github.com/libsdl-org/SDL/blob/main/sdl2-config.cmake.in#L74-L79) from sdl2-config.cmake file.