webadmin: hide migration for removing replicated bricks

'Migrate Data' option in 'Remove Bricks' dilog
will not be shown if the bricks are not from
same sub-volume (Reducing replica count).

Change-Id: Ia7fd98c0b262deff26bf9e77839ee347d5869189
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>