scripts: objdiff: remove unnecessary code

The directories for objdump is created by the code
a few lines below:

  [ ! -d "$OBJDIFFD/$dn" ] && mkdir -p "$OBJDIFFD/$dn"

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>