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

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.