As of this post, you can only hide the new store/library tabs by going into:
User Settings > App Settings > Appearance > Advanced > Show Games Tab
While this does turn off the store/library tabs, unfortunately, this also gets rid of the activity tab. The activity tab I find incredibly useful for seeing different voice channels that my friends are currently in throughout my many, many servers. Also, the game news presented there is interesting.
Being able to even hide the store at all is super nifty, and I give Discord props for giving us that option. However, of the activity, store, and library tabs, it would be awesome if we could simply toggle each individual tab on/off from the appearance settings instead of all three being group together in the "Show Games Tab" toggle.