core: Remove check for force flag when validating remove VM in preview.

There is no need to check force flag when validating if VM is in preview.

Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1071912
Change-Id: I9c43440ef999bdbb573f6038a2f6b5d01411f2ea