Revert "Add GROUP BY ALL".

Revert "Add GROUP BY ALL".

This reverts commit ef38a4d9756db9ae1d20f40aa39f3cf76059b81a which
implemented the GROUP BY ALL syntax, as well as 2ce745836 which
made some comment improvements therein.

A postcommit review discovered that GROUP BY ALL missed our special
handling of entries that also appear in an ORDER BY in the query.
This caused the query to return wrong results when ORDER BY
specifies non-default equality semantics.  While this should be
fixable with some refactoring, doing it cleanly seems like too
much code churn for late beta.  We'll revert and try again in v20.

The reverted commit also included some additional comment wordsmithing
and docs cleanup, which are retained as they weren't connected to the
reverted feature.

catversion bump needed due to change in struct Query.

Reported-by: Chao Li <li.evan.chao@gmail.com>
Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/5243308F-8E5C-45AA-828C-FAD96C4F34DA@gmail.com
Backpatch-through: 19