py/mkrules.mk: Remove special check for "-B" in qstr auto generation.

When make is passed "-B" it seems that everything is considered out-of-date
and so $? expands to all prerequisites.  Thus there is no need for a
special check to see if $? is emtpy.

py/mkrules.mk

@@ -71,12 +71,7 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
 
 $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) | $(HEADER_BUILD)/mpversion.h
 	$(ECHO) "GEN $@"
-	$(Q)if [ "$?" = "" ]; then \
-	    echo "QSTR Looks like -B used, trying to emulate"; \
-	    $(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $^ >$(HEADER_BUILD)/qstr.i.last; \
-	else \
-	    $(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $? >$(HEADER_BUILD)/qstr.i.last; \
-	fi
+	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $? >$(HEADER_BUILD)/qstr.i.last;
 
 $(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
 	$(ECHO) "GEN $@"