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>