rgw: Simplify log shard probing and err on the side of omap

rgw: Simplify log shard probing and err on the side of omap

In the multigeneration version we no longer care whether entries
exist, since we never delete and recreate empty logs. Remove logic
that marked entirely empty shards as DNE under the assumption that
they would be deleted if so.

Fixes: https://tracker.ceph.com/issues/50169
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
(cherry picked from commit 71ae2dd27ae07e6a5d2fda12834158fd2720bd7b)
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
Conflicts:
	src/rgw/rgw_log_backing.cc
- Lambda capture, changed to be strictly conforming.