LU-16518 mgs: remove dt2msg_obj/dev() and friends

LU-16518 mgs: remove dt2msg_obj/dev() and friends

Several of these helper functions are helping no one.
Let's remove them from the header.

Fixes: ef0b61b01f0f ("LU-1301 mgs: lu device for mgs")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Idf3a5dfa4549f6882e9c195301f58b90f86bee52
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58553
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>