Remove select(2) backed latch implementation.

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