crimson/seastore: segment_manager: fix -Wunused warnings

crimson/seastore: segment_manager: fix -Wunused warnings

7f739adae2 dropped the last log call from get_segment_manager(), after
which `LOG_PREFIX(SegmentManager::get_segment_manager)` and
`SET_SUBSYS(seastore_device)` had no remaining users under `HAVE_ZNS`,
generating:

```
src/crimson/os/seastore/segment_manager.cc:38:3: warning: unused variable 'FNAME' [-Wunused-variable]
   38 |   LOG_PREFIX(SegmentManager::get_segment_manager);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/crimson/common/log.h:49:38: note: expanded from macro 'LOG_PREFIX'
   49 | #define LOG_PREFIX(x) constexpr auto FNAME = #x
      |                                      ^~~~~
src/crimson/os/seastore/segment_manager.cc:10:1: warning: unused variable 'SOURCE_SUBSYS' [-Wunused-const-variable]
   10 | SET_SUBSYS(seastore_device);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~
src/crimson/common/log.h:46:52: note: expanded from macro 'SET_SUBSYS'
   46 | #define SET_SUBSYS(subname_) static constexpr auto SOURCE_SUBSYS = ceph_subsys_##subname_
      |                                                    ^~~~~~~~~~~~~
2 warnings generated.
```

drop both to silence them.

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