LU-7105 tests: remove deprecated test_28 from sanityn

LU-7105 tests: remove deprecated test_28 from sanityn

sanityn.sh test_28 was deprecated in 2022-06 but not removed.

Test-Parameters: testlist=sanityn
Fixes: 51c491dac6 ("LU-10994 test: remove netdisk from obdfilter-survey")
Signed-off-by: Frederick Dilger <fdilger@whamcloud.com>
Change-Id: I524adf575170ae9e78dc1eb5e0e1596ee7252dfe
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59052
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>