messages,osd: remove MPGStats::had_map_for

messages,osd: remove MPGStats::had_map_for

MPGStats::had_map_for was added back in 7844d0e5, the last release that
still checks this field was mimic -- monitor sends OSD incremental
osdmaps if the monitor finds that the pg_stats' had_map_for is greater
than 30 and the epoch is less than that of latest osdmap.

but DaemonServer as the new consumer of MPGStats does not check
had_map_for anymore -- it simply updates the cluster state with the
pg_stats reported by OSD. and we directs OSD to mgr for sending pg_stats
since mimic. so, we can safely drop the support of had_map_for in
octopus, as it has been 2 releases.

Signed-off-by: Kefu Chai <kchai@redhat.com>