Remove select(2) backed latch implementation.

poll(2) is required by Single Unix Spec v2, the usual baseline for
postgres (leaving windows aside).  There's not been any buildfarm
animals without poll(2) for a long while, leaving the select(2)
implementation to be largely untested.

On windows, including mingw, poll() is not available, but we have a
special case implementation for windows anyway.

Author: Andres Freund
Discussion: https://postgr.es/m/20170420003611.7r2sdvehesdyiz2i@alap3.anarazel.de