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

This reverts commit 176c70b9f2ce382a5590e42afd7ba4edb87efecb.

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 6ed3c5f7b2846be9e176ea1c99b2d45bba0d6972.