Remove contrib/tsearch2.

This module was intended to ease migrations of applications that used
the pre-8.3 version of text search to the in-core version introduced
in that release.  However, since all pre-8.3 releases of the database
have been out of support for more than 5 years at this point, we
expect that few people are depending on it at this point.  If some
people still need it, nothing prevents it from being maintained as a
separate extension, outside of core.

Discussion: http://postgr.es/m/CA+Tgmob5R8aDHiFRTQsSJbT1oreKg2FOSBrC=2f4tqEH3dOMAg@mail.gmail.com

src/test/modules/test_parser/test_parser.c

@@ -28,7 +28,6 @@ typedef struct
 	int			pos;			/* position of the parser */
 } ParserState;
 
-/* copy-paste from wparser.h of tsearch2 */
 typedef struct
 {
 	int			lexid;