LU-17995 osd-zfs: remove server_name_is_ost()

LU-17995 osd-zfs: remove server_name_is_ost()

We can determine whether a server name is for an
OST by checking the return code of server_name2index().

Test-Parameters: trivial
Test-Parameters: trivial fstype=zfs
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I2e1c337b9095d333772f87bd2a5253966b54bd45
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58163
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>