Disable helm-quick-update

It is not needed anymore since helm-candidate-number-limit is
small (less than several thousands). If this is the case, then there's
no difference between quick update and no quick update execution time.
Enabling helm-quick-update makes helm buffer flashing for every entered
character to retrieve new candidate list, which would annoy user.
Merge pull request #741 from tuhdo/master

Disable helm-quick-update