Drop server support for FE/BE protocol version 1.0.

While this isn't a lot of code, it's been essentially untestable for
a very long time, because libpq doesn't support anything older than
protocol 2.0, and has not since release 6.3.  There's no reason to
believe any other client-side code still uses that protocol, either.

Discussion: <2661.1475849167@sss.pgh.pa.us>