LU-12029 utils: remove max_sectors_kb tuning

LU-12029 utils: remove max_sectors_kb tuning

Remove max_sectors_kb tuning and make the mount option print a warning
message that it is ignored.

Remove conf-sanity test_125 which is to verify max_sectors_kb tuning.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I5d11310589f70716df98101c62fd459dafbfddca
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57278
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>