Revert "Make the default category for discussions 'uncategorized' if nothing was entered"

This reverts commit cd80abbac080c7ad89158bb57967c2b5745c23d9.

A blank category field will pass a blank string, not null, so this doesn't do much. Commit
5a054c18352cf5c5293404c2c0144eb9a1ff24ca provides a better way of handling this, which would
also better allow for changing the "uncategorized" string if desired.