exim: drop wiki-mail from WIKI_INTERFACE

exim is not smart enough to pick the right address; mchenry is not
puppetized anyway, so dropping the (pmtpa) wiki-mail address is a noop
anyway, especially since this address is going to be deprecated very
soon now.

Change-Id: I3e213d512d9aee93c9992a8bd44fa1911d2bc82b