kbuild: drop unused symverfile in Makefile.modpost

Since commit 040fcc81 ("kbuild: improved modversioning
support for external modules"), symverfile has been replaced
with kernelsymfile and modulesymfile.
Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.modpost

@@ -97,7 +97,6 @@ vmlinux.o: FORCE
 	$(call cmd,kernel-mod)
 
 # Declare generated files as targets for modpost
-$(symverfile):         __modpost ;
 $(modules:.ko=.mod.c): __modpost ;