Remove grammar productions for prefix and postfix % and ^ operators,

as well as the existing pg_catalog entries for prefix and postfix %.
These have never been documented, though they did appear in one old
regression test.  This avoids surprising behavior in cases like
"SELECT -25 % -10".  Per recent discussion.
Note: although there is a catalog change here, I did not force initdb
since there's no harm in leaving the inaccessible entries in one's
copy of pg_operator.