mgr/rbd_support: always parse interval and start_time in Schedules::remove()

mgr/rbd_support: always parse interval and start_time in Schedules::remove()

Commit 1b62447071a9 ("mgr/rbd_support: fix schedule remove") addressed
the issue that it was concerned with in a rather suboptimal way: instead
of moving the parsing of interval and start_time upfront to be able to
bail early, it wrapped from_string() constructors with try/finally and
left the conditional behavior in place.

Fixes: https://tracker.ceph.com/issues/70640
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit bcd676356231c97c8275e6c03becc2c58497f20d)