Remove gram.y's precedence declaration for OVERLAPS.

The allowed syntax for OVERLAPS, viz "row OVERLAPS row", is sufficiently
constrained that we don't actually need a precedence declaration for
OVERLAPS; indeed removing this declaration does not change the generated
gram.c file at all.  Let's remove it to avoid confusion about whether
OVERLAPS has precedence or not.  If we ever generalize what we allow for
OVERLAPS, we might need to put back a precedence declaration for it,
but we might want some other level than what it has today --- and leaving
the declaration there would just risk confusion about whether that would
be an incompatible change.

Likewise, remove OVERLAPS from the documentation's precedence table.

Per discussion with Noah Misch.  Back-patch to 9.5 where we hacked up some
nearby precedence decisions.