Fix 0752 and 0756 upgrade scripts

In 0752, change evergreen.is_json() to just is_json() since some
upgraded sites may still have this in their public schema.

In 0756, drop some additional remnant functions if they exist. These
functions may still lurk in older Evergreen systems upgrading from
pre-2.0 times.

See for reference these commits which drop the older functions from
the schema:

415e0c40cf8ac0fbba91541f9ddf386d02f01fec
24007bf27d6f53073cfb45e4db43b88f651b971b

Signed-off-by: Ben Shum <bshum@biblio.org>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>