core: cleanup in RemoveVmCommand class

- Remove canRemoveVmWithDetachDisks#isSnapshotsPresent field
- Extract the parameters construction in removeVmImages method to
  separate method

Change-Id: I4079a58d39c613a55c832c62831e78767580842a
Signed-off-by: Arik Hadas <ahadas@redhat.com>