pgsql: Remove server support for the previous base backup protocol.

Remove server support for the previous base backup protocol.

Commit cc333f32336f5146b75190f57ef587dff225f565 added a new COPY
sub-protocol for taking base backups, but retained support for the
previous protocol. For the same reasons articulated in the message
for commit 9cd28c2e5f11dfeef64a14035b82e70acead65fd, remove support
for the previous protocol from the server.

Discussion: http://postgr.es/m/CA+TgmoazKcKUWtqVa0xZqSzbKgTH+X-aw4V7GyLD68EpDLMh8A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d4513b6138650355fcc981a9f34ff57b8d69db4

Modified Files
--------------
doc/src/sgml/protocol.sgml                |  34 ++----
src/backend/replication/basebackup.c      |  11 +-
src/backend/replication/basebackup_copy.c | 172 ++----------------------------
src/include/replication/basebackup_sink.h |   1 -
4 files changed, 21 insertions(+), 197 deletions(-)