Remove quantum.h includes from keyboard custom matrix.cs (#23371)

keyboards/mt/split75/matrix.c

@@ -15,9 +15,7 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include <string.h>
-#include <stdio.h>
-#include "quantum.h"
+#include "matrix.h"
 #include "i2c_master.h"
 
 #define RIGHT_HALF