As far as I know, you can't.
That list is "every available class" connected to your page, aside from maybe commenting out the bootstrap .css file temporarily, I think you're stuck with "all".
It's much easier to use Code View's Code Hinting, or the Live View Editor's "HUD" to add classes...
1. Click the element in Live or Design to select it in Code View
2. Add a class attribute, or click within an existing one and start typing the class name. DW will limit the classes in the list to those starting with what you've typed.
1. Click the element you want to add a class to in Live View
2. Within the blue tab that appears on the top left of the element, click the + icon
3. Start typing the class name in the field that appears, the code hinting will limit the enormous class list as you type
This is another area where Adobe went off the rails when they moved away from DWCC2015. You used to be able to flick the scroll wheel to move through that gigantic list, or type the first letter of the name of the class to be brought to that section of the list alphabetically.
That's gone now, making the pop up list in the Properties window completely useless in CC2018.
Personally, I won't use CC2018 for production work, it still has far too many problems. I leave it installed along side CC2015, hopefully Adobe can squash all of the bugs at some point and I can update in the future.
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