fix CheckCOCOA on modern CMake

Modern CMake doesn't need "LANGUAGE C" for Objective-C
set_source_files_properties(${COCOA_SOURCES} PROPERTIES LANGUAGE C) should be removed fro video/cocoa/*.m as well otherwise project won't compile
see also https://github.com/libsdl-org/SDL/commit/d3cc5764c0f66872a6b87c0ffbca66c85dccbea3

cmake/sdlchecks.cmake

@@ -711,7 +711,6 @@ macro(CheckCOCOA)
     endif()
     if(HAVE_COCOA)
       file(GLOB COCOA_SOURCES ${SDL2_SOURCE_DIR}/src/video/cocoa/*.m)
-      set_source_files_properties(${COCOA_SOURCES} PROPERTIES LANGUAGE C)
       set(SOURCE_FILES ${SOURCE_FILES} ${COCOA_SOURCES})
       set(SDL_VIDEO_DRIVER_COCOA 1)
       set(HAVE_SDL_VIDEO TRUE)