kbuild: drop $(version_h) from MRPROPER_FILES

Now $(version_h) is include/generated/uapi/linux/version.h.

$(version_h) in MRPROPER_FILES is redundant because it is covered
by include/generated in MRPROPER_DIRS.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>