osd/pg_scrubber: do not print log in dtors

osd/pg_scrubber: do not print log in dtors

PG::recovery_state is defined after PG::m_scrubber, while
PG::gen_prefix() retrieves the osdmap from recovery_state. in
PgScrubber::~PgScrubber(), we print out the osdmap by dereferencing
recovery_state for the prefix of the logging message. in other words,
PgScrubber's destructor is referencing "recovery_state" which is
already destroyed.

in this change, the logging messages are removed from dtors. we can
print out the pgid in a follow-up PR if it helps.

Fixes: https://tracker.ceph.com/issues/48777
Signed-off-by: Kefu Chai <kchai@redhat.com>