gcode_process.c: remove G30.

This was "Go home via point". The RepRap community has apparently
decided for a super complex Z probing command with this number:

  http://reprap.org/wiki/G-code#G30:_Single_Z-Probe

This reduces binary size by 18 bytes:

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19508 bytes      137%       64%       31%       16%
     Data:   2175 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

gcode_process.c

@@ -167,13 +167,6 @@ void process_gcode_command() {
 				next_target.option_inches = 0;
 				break;
 
-			case 30:
-				//? --- G30: Go home via point ---
-				//?
-				//? Undocumented.
-				enqueue(&next_target.target);
-				// no break here, G30 is move and then go home
-
 			case 28:
 				//? --- G28: Home ---
 				//?