Remove M999 remnants

This was already removed in fc10ca31466e1c58bcb5b9b90976a489239a6fde
but got reinstated by mistake in the mmu2 merge.

Also remove another incorrect Stopped usage.

Firmware/Marlin_main.cpp

@@ -8657,16 +8657,6 @@ Sigma_Exit:
     }
     break;
 
-    /*!
-    ### M999 - Restart after being stopped <a href="https://reprap.org/wiki/G-code#M999:_Restart_after_being_stopped_by_error">M999: Restart after being stopped by error</a>
-    @todo Usually doesn't work. Should be fixed or removed. Most of the time, if `Stopped` it set, the print fails and is unrecoverable.
-    */
-    case 999:
-      Stopped = false;
-      lcd_reset_alert_level();
-//@@TODO      gcode_LastN = Stopped_gcode_LastN;
-      FlushSerialRequestResend();
-    break;
 	/*!
 	#### End of M-Commands
     */

Firmware/cardreader.cpp

@@ -273,7 +273,6 @@ void CardReader::startFileprint()
   if(cardOK)
   {
     sdprinting = true;
-    Stopped = false;
 	#ifdef SDCARD_SORT_ALPHA
 		//flush_presort();
 	#endif