crimson/osd: remove OSD::startup_time

OSD::startup_time was added in 91c0df81, in which was added to provide
a monotomic increasing timestamp representing the startup time. but
later, we decided to keep track of this timestamp in PerShardState.
but we didn't remove OSD::startup_time when adding
PerShardState::get_mnow().

in this change, we remove the unused OSD::startup_time.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>