crimson/osd: drop redundant code

crimson/osd: drop redundant code

this change is a cleanup.

we already update `superblock` with the latest `boost_epoch` and
osdmap's epoch in `OSD::handle_osd_map()`, and `committed_osd_maps()` is
called at end of this function. `shutdown()` is called when we marked
down and stop by the monitor in `committed_osd_maps()`. so these
assignment statements are noops. instead, we should stop the whole osd
service. let's leave it for another commit.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>