test_stress_watch: remove bogus asserts

There's no reason to check the duration of a watch. The notify will
timeout after 30s on the OSD, but there's no guarantee the client will
see that in any bounded time. This test is really meant as a stress
test of the OSDs anyway, not of the clients, so just remove asserts
about operation duration.

Fixes: #4591
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sam Just <sam.just@inktank.com>