LU-17848 dt: allow declare functions to be optional

LU-17848 dt: allow declare functions to be optional

If an OSD (or other dt implementer) doesn't have anything to
declare, don't force it to implement a declare function for
an operation.

Clean up some examples of useless declare functions in osp
and lfsck.

Test-Parameters: trivial
Test-Parameters: trivial fstype=zfs
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I0d7e9f491ff2a8f6e4f3bf315a10437cd42c2351
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55632
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>