Change keyboard level include guards to pragma once (#14248)

* Change keyboard level include guards to `pragma once`

And clean up a lot of layout macros

* Oops

* Remove dangling endif

keyboards/mechmini/mechmini.h

@@ -14,17 +14,12 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#ifndef MECHMINI_H
-#define MECHMINI_H
+#pragma once
 
 #include "quantum.h"
 
-#ifdef KEYBOARD_mechmini_v1
-    #include "v1.h"
-#endif
-
-#ifdef KEYBOARD_mechmini_v2
-    #include "v2.h"
-#endif
-
+#if defined(KEYBOARD_mechmini_v1)
+#    include "v1.h"
+#elif defined(KEYBOARD_mechmini_v2)
+#    include "v2.h"
 #endif