Remove SUBSYS.o rule in common.mk, hasn't been used in a long time

Apparently I missed that this SUBSYS.o rule isn't needed anymore in
a4ebbd2752, likely because there still is a reference to it due to AIX - but
that's self contained in src/backend/Makefile

Discussion: https://postgr.es/m/20220820174213.d574qde4ptwdzoqz@awork3.anarazel.de

src/backend/common.mk

@@ -17,9 +17,6 @@ ifneq ($(subdir), src/backend)
 all: $(subsysfilename)
 endif
 
-SUBSYS.o: $(SUBDIROBJS) $(OBJS)
-	$(LD) $(LDREL) $(LDOUT) $@ $^
-
 objfiles.txt: Makefile $(SUBDIROBJS) $(OBJS)
 # Don't rebuild the list if only the OBJS have changed.
 	$(if $(filter-out $(OBJS),$?),( $(if $(SUBDIROBJS),cat $(SUBDIROBJS); )echo $(addprefix $(subdir)/,$(OBJS)) ) >$@,touch $@)