Remove tab completion for CREATE UNLOGGED MATERIALIZED VIEW.

Remove tab completion for CREATE UNLOGGED MATERIALIZED VIEW.

Commit 3bf3ab8c56 added support for unlogged materialized views,
but commit 3223b25ff7 reverted that feature before it made it into
a release.  However, the latter commit left the grammar and
tab-completion support intact.  This commit removes the
tab-completion support to prevent psql from recommending bogus
commands.  I've opted to keep the grammar support so that the
server continues to emit a descriptive error when users try to
create unlogged matviews.

Reported-by: Daniel Westermann, px shi
Author: Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/ZR0P278MB092093E92263DE16734208A5D2C59%40ZR0P278MB0920.CHEP278.PROD.OUTLOOK.COM
Discussion: https://postgr.es/m/CAAccyY%2BWg1Z-9tNfSwLmuZVgGOwqU5u1OP-RWcoAr2UZGuvN_w%40mail.gmail.com