kbuild: mkmakefile: do not check the generated Makefile marker

This hunk was added to avoid accidental overwrite of the top Makefile
in case O= points to the top of the source tree.

As commit 4f1127e2 ("kbuild: fix infinite make recursion"),
it caused some troubles in the past because Kbuild assumes O=
as out-of-tree build, while it actually works in the source tree.

Now this works more properly; if O= points to the source directory,
it is handled as in-tree build. So, this sanity check is unneeded.

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