allow executemany values for ON CONFLICT DO NOTHING

allow executemany values for ON CONFLICT DO NOTHING

Scaled back a fix made for :ticket:`6581` where "executemany values" mode
for psycopg2 were disabled for all "ON CONFLICT" styles of INSERT, to
not apply to the "ON CONFLICT DO NOTHING" clause, which does not include
any parameters and is safe for "executemany values" mode.  "ON CONFLICT
DO UPDATE" is still blocked from "executemany values" as there may
be additional parameters in the DO UPDATE clause that cannot be batched
(which is the original issue fixed by :ticket:`6581`).

Fixes: #7880
Change-Id: Id3e23a0c6699333409a50148fa8923cb8e564bdc