kbuild: link-vmlinux.sh: simplify .version increment

Since commit 1f2bfbd0 ("kbuild: link of vmlinux moved to a
script"), it is easy to increment .version without using a temporary
file .old_version.

I do not see anybody who creates the .tmp_version.  Probably it is a
left-over of commit 4e25d8bb ("[PATCH] kbuild: adjust .version
updating").  Just remove it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>