Don't show remove reviewer button on closed changes

If the change is closed, we shouldn't be able to delete a reviewer
from the reviewer table.  So don't display the button to the user.

Change-Id: I1b4c7e3f2f764a7109fba3bbd93cef9ce4d2622e
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed