osd: clear recovery state on pg removal

This ensures we release our in-progress recovery counters, which prevents
recovery from getting blocked indefinitely when a pool removal races with
recovery ops.

Fixes: #4217
Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 6d8dfb18feb4ce263453039fc0afdb1462279e9a)