Remove dead code and fix comments in fast-path function handling.

HandleFunctionRequest() is no longer responsible for reading the protocol
message from the client, since commit 2b3a8b20. Fix the outdated
comments.

HandleFunctionRequest() now always returns 0, because the code that used
to return EOF was moved in 2b3a8b20. Therefore, the caller no longer
needs to check the return value.

Reported by Andres Freund. Backpatch to all supported versions, even though
this doesn't have any user-visible effect, to make backporting future
patches in this area easier.

Discussion: https://www.postgresql.org/message-id/20170405010525.rt5azbya5fkbhvrx@alap3.anarazel.de