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