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

drivers/avr/i2c_slave.h

@@ -20,8 +20,7 @@
        Read or write to the necessary buffer according to the opperation.
  */
 
-#ifndef I2C_SLAVE_H
-#define I2C_SLAVE_H
+#pragma once
 
 #define I2C_SLAVE_REG_COUNT 30
 
@@ -29,5 +28,3 @@ extern volatile uint8_t i2c_slave_reg[I2C_SLAVE_REG_COUNT];
 
 void i2c_slave_init(uint8_t address);
 void i2c_slave_stop(void);
-
-#endif  // I2C_SLAVE_H