pgsql: Some minor code cleanup, falling out from the removal of rtree.

Log Message:
-----------
Some minor code cleanup, falling out from the removal of rtree.  SK_NEGATE
isn't being used anywhere anymore, and there seems no point in a generic
index_keytest() routine when two out of three remaining access methods
aren't using it.  Also, add a comment documenting a convention for
letting access methods define private flag bits in ScanKey sk_flags.
There are no such flags at the moment but I'm thinking about changing
btree's handling of "required keys" to use flag bits in the keys
rather than a count of required key positions.  Also, if some AM did
still want SK_NEGATE then it would be reasonable to treat it as a private
flag bit.

Modified Files:
--------------
    pgsql/src/backend/access/common:
        Makefile (r1.20 -> r1.21)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/Makefile.diff?r1=1.20&r2=1.21)
    pgsql/src/backend/access/gist:
        gistget.c (r1.54 -> r1.55)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c.diff?r1=1.54&r2=1.55)
    pgsql/src/backend/access/hash:
        hashutil.c (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashutil.c.diff?r1=1.44&r2=1.45)
    pgsql/src/backend/executor:
        execUtils.c (r1.131 -> r1.132)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execUtils.c.diff?r1=1.131&r2=1.132)
    pgsql/src/include/access:
        skey.h (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/skey.h.diff?r1=1.29&r2=1.30)

Removed Files:
-------------
    pgsql/src/backend/access/common:
        indexvalid.c
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/indexvalid.c)
    pgsql/src/include/access:
        iqual.h
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/iqual.h)