s390/purgatory: remove duplicated build rule of kexec-purgatory.o

This is equivalent to the pattern rule in scripts/Makefile.build.

Having the dependency on $(obj)/purgatory.ro is enough.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220613170902.1775211-3-masahiroy@kernel.orgSigned-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/purgatory/Makefile

@@ -48,7 +48,6 @@ OBJCOPYFLAGS_purgatory.ro += --remove-section='.note.*'
 $(obj)/purgatory.ro: $(obj)/purgatory $(obj)/purgatory.chk FORCE
 		$(call if_changed,objcopy)
 
-$(obj)/kexec-purgatory.o: $(obj)/kexec-purgatory.S $(obj)/purgatory.ro FORCE
-	$(call if_changed_rule,as_o_S)
+$(obj)/kexec-purgatory.o: $(obj)/purgatory.ro
 
 obj-y += kexec-purgatory.o