Disable preview for tab completion

[daephx]

With my setup the preview window for tab completion is too vertically narrow to be of any real use and I am not a fan of it being enabled by default.

Looking around the docs and code It's not clear to me if this is even possible with the current implementation.
If this is in-fact possible, I would appreciate some pointers. Otherwise I think this is a issue.

[yanghanlin]

Same issue here. The preview window causes some lag on my machine, but I can't find any option to disable it.