Apply #pragma once, misc cleanup (#12322)

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

Marlin/src/libs/least_squares_fit.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * Incremental Least Squares Best Fit  By Roxy and Ed Williams
@@ -32,9 +33,6 @@
  *
  */
 
-#ifndef _LEAST_SQUARES_FIT_H_
-#define _LEAST_SQUARES_FIT_H_
-
 #include "../inc/MarlinConfig.h"
 #include <math.h>
 
@@ -83,5 +81,3 @@ void inline incremental_LSF(struct linear_fit_data *lsf, const float &x, const f
 }
 
 int finish_incremental_LSF(struct linear_fit_data *);
-
-#endif // _LEAST_SQUARES_FIT_H_