genext2fs: remove it

Remove genext2fs since we don't use it anymore, it can't support
ext4 well, either. We have used "mke2fs -d" to instead of it.

[YOCTO #6013]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>