Fix API list=deletedrevs continuation in mode 1

r86782 introduced validation of parameter combinations for
list=deletedrevs, but screwed up in that it disallowed drcontinue in
"mode 1" (list all deleted revisions for the specified titles).

Apparently no one uses this much, because it doesn't seem to have been
reported as a bug despite being broken since April 2011.

Change-Id: I30691d04fab759ee56fb4156d475fbb21b81ab45