Adjust citext to make use of the new ability to declare its type category:

Adjust citext to make use of the new ability to declare its type category:
by putting it into the standard string category, we cause casts from citext
to text to be recognized as "preferred" casts.  This eliminates the need
for creation of alias functions and operators that only serve to prevent
ambiguous-function errors; get rid of the ones that were in the original
commit.