Move PLPERL dropping outside of 2.1-2.2 upgrade transaction

Move PLPERL dropping outside of 2.1-2.2 upgrade transaction

Sites might have added custom PLPERL database functions for migrations,
data clean up, etc, so don't make the success of the 2.1-2.2 upgrade
hinge on a database schema that exactly matches vanilla Evergreen;
just move it outside of the upgrade transaction and output a reassuring
note.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>