Revert MERGE patch

This reverts commits d204ef63,
83454e3c and a few more commits thereafter
(complete list at the end) related to MERGE feature.

While the feature was fully functional, with sufficient test coverage and
necessary documentation, it was felt that some parts of the executor and
parse-analyzer can use a different design and it wasn't possible to do that in
the available time. So it was decided to revert the patch for PG11 and retry
again in the future.

Thanks again to all reviewers and bug reporters.

List of commits reverted, in reverse chronological order:

 f1464c53 Improve parse representation for MERGE
 ddb41585 MERGE syntax diagram correction
 530e69e5 Allow cpluspluscheck to pass by renaming variable
 01b88b4d MERGE minor errata
 3af7b2b0 MERGE fix variable warning in non-assert builds
 a5d86181 MERGE INSERT allows only one VALUES clause
 4b2d4403 MERGE post-commit review
 4923550c Tab completion for MERGE
 aa3faa3c WITH support in MERGE
 83454e3c New files for MERGE
 d204ef63 MERGE SQL Command following SQL:2016

Author: Pavan Deolasee
Reviewed-by: Michael Paquier