pgsql: Remove server support for old BASE_BACKUP command syntax.

Remove server support for old BASE_BACKUP command syntax.

Commit 0ba281cb4bf9f5f65529dfa4c8282abb734dd454 added a new syntax
for the BASE_BACKUP command, with extensible options, but maintained
support for the legacy syntax. This isn't important for PostgreSQL,
where pg_basebackup works with older server versions but not newer
ones, but it could in theory matter for out-of-core users of the
replication protocol.

Discussion on pgsql-hackers, however, suggests that no one is aware
of any out-of-core use of the BASE_BACKUP command, and the consensus
is in favor of removing support for the old syntax to simplify the
code, so do that.

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/9cd28c2e5f11dfeef64a14035b82e70acead65fd

Modified Files
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doc/src/sgml/protocol.sgml             | 11 ----
src/backend/replication/repl_gram.y    | 97 ++--------------------------------
src/backend/replication/repl_scanner.l | 10 ----
3 files changed, 4 insertions(+), 114 deletions(-)