rgw: fix log_remove spurious -EIO on non-existent FIFO shards

the shared error code `ec` was not cleared before continuing
past a non-existent shard (ENOENT from get_meta). and if the last
shard returned ENOENT, `ec` retained that value and the
fired EIO even though all shards were cleanly removed.

also switch to sys::system_error(ec) to preserve the real errno

Signed-off-by: Shilpa Jagannath <smanjara@redhat.com>