kbuild: drop redundant "PHONY += FORCE"

"PHONY += FORCE" is already cared by scripts/Makefile.build,
which these files are included from.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.com>
diff --git a/arch/ia64/Makefile b/arch/ia64/Makefile
index 648f1ce..c100d78 100644
--- a/arch/ia64/Makefile
+++ b/arch/ia64/Makefile
@@ -96,7 +96,7 @@
 endef
 
 archprepare: make_nr_irqs_h
-PHONY += make_nr_irqs_h FORCE
+PHONY += make_nr_irqs_h
 
 make_nr_irqs_h:
 	$(Q)$(MAKE) $(build)=arch/ia64/kernel include/generated/nr-irqs.h