kbuild: remove duplicated dependency between modules and modules_check
The dependency, "modules: modules_check" is specified twice.
Commit 1a998be620a1 ("kbuild: check module name conflict for external
modules as well") missed to clean it up.
'PHONY += modules' also appears twice.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>