🧑‍💻 Simplify some flag params

Marlin/src/gcode/calibrate/G76_M871.cpp

@@ -320,7 +320,7 @@
  */
 void GcodeSuite::M871() {
 
-  if (parser.seen('R')) {
+  if (parser.seen_test('R')) {
     // Reset z-probe offsets to factory defaults
     ptc.clear_all_offsets();
     SERIAL_ECHOLNPGM("Offsets reset to default.");

Marlin/src/gcode/feature/cancel/M486.cpp

@@ -47,7 +47,7 @@ void GcodeSuite::M486() {
   if (parser.seenval('S'))
     cancelable.set_active_object(parser.value_int());
 
-  if (parser.seen('C')) cancelable.cancel_active_object();
+  if (parser.seen_test('C')) cancelable.cancel_active_object();
 
   if (parser.seenval('P')) cancelable.cancel_object(parser.value_int());