Allow configuration to not use default RouteVoter

[kopeckyales]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no

It would be nice to be able to disable registering of the default RouteVoter provided by knp.

Example situation is when one has really complex menu structure and performance optimized voter for it, but default RouteVoter is always called for every item, when user voter returns null, because for some reason can not decide about given item (another user voter exists e.g).