Apply #pragma once, misc cleanup (#12322)

* Apply #pragma once in headers
* Adjust some thermistors formatting
* Misc cleanup and formatting

Marlin/src/gcode/gcode.h

@@ -251,9 +251,7 @@
  * T0-T3 - Select an extruder (tool) by index: "T<n> F<units/min>"
  *
  */
-
-#ifndef _GCODE_H_
-#define _GCODE_H_
+#pragma once
 
 #include "../inc/MarlinConfig.h"
 #include "parser.h"
@@ -829,5 +827,3 @@ private:
 };
 
 extern GcodeSuite gcode;
-
-#endif // _GCODE_H_

Marlin/src/gcode/queue.h

@@ -19,15 +19,13 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * queue.h - The G-code command queue, which holds commands before they
  *           go to the parser and dispatcher.
  */
 
-#ifndef GCODE_QUEUE_H
-#define GCODE_QUEUE_H
-
 #include "../inc/MarlinConfig.h"
 
 /**
@@ -125,5 +123,3 @@ void get_available_commands();
  * Get the next command in the queue, optionally log it to SD, then dispatch it
  */
 void advance_command_queue();
-
-#endif // GCODE_QUEUE_H