Remove dialog from rescue Makefile.

dialog introduces unwanted dependencies in rescue.
If we want to add a binary to the installation ISO,
we should explicitly add it to the scripts which create
the install ISO, and not modify src/rescue/rescue/Makefile.

In FreeNAS, we cleaned up the create_iso.sh script in these changesets:
d29cbfcc8e9bed8510b59dd86e3e93f3c0e00ea2
db5e03a1451c6e3cac5bb3765897feda6ab940be

so that the install ISO takes dialog from /usr/bin/dialog and *not* /rescue/dialog.