PG: clear want_pg_temp in clear_primary_state only if primary

Clearing it in that way in on_shutdown() can cause a stray
shard to clobber the want_pg_temp value created by the primary
shard on the same osd.  Thus, instead only clear it if we are
the primary.

Fixes: #7719
Signed-off-by: Samuel Just <sam.just@inktank.com>