core: cleanup RemoveVmTemplateCommand#canDoAction

1. Replace consecutive lines of call to addCanDoActionMessage and then
'return false' with 'return failCanDoAction(..)'
2. Replace calls to DbFacade.getInstance() with call to getDbFacade()
3. Replace if-else statement with ternary-if

Change-Id: If669283fc057cff4eee46dfb15e004672d05fac1
Signed-off-by: Arik Hadas <ahadas@redhat.com>