sdl: use default autotools target-install

Fixes http://autobuild.buildroot.org/results/c950d7920d384cb0d37403cbe8344691d4231a1a

The normal 'make install' is perfectly fine, and it also works for static
builds.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/sdl/sdl.mk

@@ -70,8 +70,4 @@ endef
 
 SDL_POST_INSTALL_STAGING_HOOKS+=SDL_FIXUP_SDL_CONFIG
 
-define SDL_INSTALL_TARGET_CMDS
-	cp -dpf $(STAGING_DIR)/usr/lib/libSDL*.so* $(TARGET_DIR)/usr/lib/
-endef
-
 $(eval $(autotools-package))