Remove vestigial grammar support for CHARACTER ... CHARACTER SET option.

Remove vestigial grammar support for CHARACTER ... CHARACTER SET option.

The SQL standard says that you should be able to write "CHARACTER SET foo"
as part of the declaration of a char-type column.  We don't implement that,
but a rough form of support has existed in gram.y since commit f10b63923.
That's now sat there for nigh 20 years without anyone fleshing it out ---
and even if someone did, the contemplated approach of having separate data
type name(s) for every character set certainly isn't what we'd do today.
Let's just remove the grammar production; if anyone is ever motivated to
work on this, reinventing the grammar support is a trivial fraction of
what they'd have to do.  And we've never documented anything about
supporting such a clause.

Per gripe from Neha Khatri.

Discussion: https://postgr.es/m/CAFO0U+-iOS5oYN5v3SBuZvfhPUTRrkDFEx8w7H17B07Rwg3YUA@mail.gmail.com