Revert accidental removal of check

Firmware/mmu2.cpp

@@ -18,6 +18,9 @@
 #include "cardreader.h" // for IS_SD_PRINTING
 #include "SpoolJoin.h"
 
+// As of FW 3.12 we only support building the FW with only one extruder, all the multi-extruder infrastructure will be removed.
+// Saves at least 800B of code size
+static_assert(EXTRUDERS==1);
 
 namespace MMU2 {