Remove useless PGRES_COPY_BOTH "support" in psql.

There is no existing or foreseeable case in which psql should see a
PGRES_COPY_BOTH PQresultStatus; and if such a case ever emerges, it's a
pretty good bet that these code fragments wouldn't do the right thing
anyway.  Remove them, and let the existing default cases do the appropriate
thing, namely emit an "unexpected PQresultStatus" bleat.

Noted while working on libpq row processor patch, for which I was
considering adding a PGRES_SUSPENDED status code --- the same default-case
treatment would be appropriate for that.