kbuild: remove user ID check in scripts/mkmakefile

This line was added by commit fd5f0cd6 ("kbuild: Do not overwrite
makefile as anohter user").  Its commit description says the intention
was to prevent $(objtree)/Makefile from being owned by root when e.g.
running 'make install'.

However, as commit 19514fc6 ("arm, kbuild: make "make install" not
depend on vmlinux") stated, installation targets must not modify the
source tree in the first place.  If they do, we are already screwed up.
We must fix the root cause.

Installation targets should just copy files verbatim, hence we never
expect $(objtree)/Makefile is touched by root.  The user ID check in
scripts/mkmakefile is unneeded.

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