Remove support for YCM.

YCM has now its own navigation functions, so there is no longer any need
to call it from syntastic.

g:syntastic_delayed_redraws is also gone.  Syntastic doesn't use popup
menus, and the only purpose of this variable was to provide a workaround
for a problem triggered by YCM.

References:
https://github.com/Valloric/YouCompleteMe/pull/763