package/sdl2: remove sdl2-config.cmake

package/sdl2: remove sdl2-config.cmake

We are using autotools build system for sdl2, so the sdl2-config.cmake
include path are not resolved like for sdl2-config script [1].
Remove sdl2-config.cmake file and avoid unsafe include path if this
file is used by a cmake based package.

This trigger an issue with ogre 1.12.6 package that replaced
FindSDL2.cmake by sdl2-config.cmake [2].

Thanks to Pavel Rojtberg for the help [3].

[1] https://bugzilla.libsdl.org/show_bug.cgi?id=4597
[2] https://github.com/OGRECave/ogre/commit/6de6f9b4081a07c911932441700c54b72ec1ee1f
[3] https://github.com/OGRECave/ogre/issues/1568

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>