LU-18753 mdt: remove mdt_buf code

LU-18753 mdt: remove mdt_buf code

Remove unused MDT buffer code, including mdt_buf()
and mdt_buf_const().

Fixes: 26b823865997 ("LU-3105 osd: remove capa related stuff from servers")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I7c0ffc820e94886902a1dbf09e01d70761f7d8fd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58228
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Max Wang <wamax@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>