Clean up remove_rel_from_query() after self-join elimination commit

Clean up remove_rel_from_query() after self-join elimination commit

The self-join elimination (SJE) commit grafted self-join removal onto
remove_rel_from_query(), which was originally written for left-join
removal only.  This resulted in several issues:

- Comments throughout remove_rel_from_query() still assumed only
  left-join removal, making the code misleading.

- ChangeVarNodesExtended was called on phv->phexpr with subst=-1
  during left-join removal, which is pointless and confusing since any
  surviving PHV shouldn't reference the removed rel.

- phinfo->ph_lateral was adjusted for left-join removal, which is
  unnecessary since the removed relid cannot appear in ph_lateral for
  outer joins.

- The comment about attr_needed reconstruction was in
  remove_rel_from_query(), but the actual rebuild is performed by the
  callers.

- EquivalenceClass processing in remove_rel_from_query() is redundant
  for self-join removal, since the caller (remove_self_join_rel)
  already handles ECs via update_eclasses().

- In remove_self_join_rel(), ChangeVarNodesExtended was called on
  root->processed_groupClause, which contains SortGroupClause nodes
  that have no Var nodes to rewrite.  The accompanying comment
  incorrectly mentioned "HAVING clause".

This patch fixes all these issues, clarifying the separation between
left-join removal and self-join elimination code paths within
remove_rel_from_query().  The resulting code is also better structured
for adding new types of join removal (such as inner-join removal) in
the future.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48JC4OVqE=3gMB6se2WmRNNfMyFyYxm-09vgpm+Vwe8Hg@mail.gmail.com