misc: Drop useless checks from *Dispose() functions

misc: Drop useless checks from *Dispose() functions

Due to the way that our virObjectUnref() is written it's not
possible that a NULL is passed into *Dispose() function. However,
some functions check for that regardless.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>