array: remove duplicate nested argcheck, correctly this time.

* lib/std/array.lua: Where a function or metamethod is called via
`dispatch`, which already checks that the first argument is an
Array object, don't recheck type of self.  If that leaves only
arguments of type "any" then don't spend any time checking
argument types at all.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>