Revert "Backpatch pgxs vpath build and installation fixes (v2)"

This reverts commit dd9abd3c995dbc4d32cfc97fde03fe3583e2717c.

pending resolution of
http://www.postgresql.org/message-id/1381193255.25702.4.camel@vanquo.pezone.net
Revert "Document support for VPATH builds of extensions."

This reverts commit 565beb41bdd73611ddb26165b7f6bb8b6157e49d.