test/librbd/test_notify.py: drop RBD_DISABLE_UPDATE_FEATURES

test/librbd/test_notify.py: drop RBD_DISABLE_UPDATE_FEATURES

This was put in place in commit 9c0b239d70cd ("qa/upgrade:
conditionally disable update_features tests") to paper over a backwards
compatibility issue that arose from commit 01ff1530544c ("librbd: make
all maintenance op notifications async").  It's not needed in squid or
later because upgrades from octopus are tested only until reef.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit bfc0b4f3015ec79c00a0ab684a4b7d58cb667e86)