Add ALTER .. NO DEPENDS ON

Commit f2fcad27d59c (9.6 era) added the ability to mark objects as
dependent an extension, but forgot to add a way for such dependencies to
be removed.  This commit fixes that oversight.

Strictly speaking this should be backpatched to 9.6, but due to lack of
demand we're not doing so at this time.

Discussion: https://postgr.es/m/20200217225333.GA30974@alvherre.pgsql
Reviewed-by: ahsan hadi <ahsan.hadi@gmail.com>
Reviewed-by: Ibrar Ahmed <ibrar.ahmad@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
