pgsql: Revert "MERGE SQL Command following SQL:2016"

Revert "MERGE SQL Command following SQL:2016"

This reverts commit e6597dc3533946b98acba7871bd4ca1f7a3d4c1d.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/aa5877bb26347c58a34aee4e460eb1e1123bb096

Modified Files
--------------
doc/src/sgml/ref/merge.sgml                        |  603 -------
src/backend/executor/nodeMerge.c                   |  575 -------
src/backend/parser/parse_merge.c                   |  660 -------
src/include/executor/nodeMerge.h                   |   22 -
src/include/parser/parse_merge.h                   |   19 -
src/test/isolation/expected/merge-delete.out       |   97 --
.../isolation/expected/merge-insert-update.out     |   84 -
.../isolation/expected/merge-match-recheck.out     |  106 --
src/test/isolation/expected/merge-update.out       |  213 ---
src/test/isolation/specs/merge-delete.spec         |   51 -
src/test/isolation/specs/merge-insert-update.spec  |   52 -
src/test/isolation/specs/merge-match-recheck.spec  |   79 -
src/test/isolation/specs/merge-update.spec         |  132 --
src/test/regress/expected/merge.out                | 1811 --------------------
src/test/regress/sql/merge.sql                     | 1173 -------------
15 files changed, 5677 deletions(-)