Change include guards in tmk_core/ and drivers/ to pragma once (#11240)

tmk_core/common/avr/timer_avr.h

@@ -15,8 +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/>.
 */
 
-#ifndef TIMER_AVR_H
-#define TIMER_AVR_H 1
+#pragma once
 
 #include <stdint.h>
 
@@ -38,5 +37,3 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #if (TIMER_RAW_TOP > 255)
 #    error "Timer0 can't count 1ms at this clock freq. Use larger prescaler."
 #endif
-
-#endif