Remove traces of opt-in mode to use improved hide heuristics, setting it as default

This issue is Step 3 of the Epic &14444.

The task is to update gitlab-ui to remove the feature flag handling again, and set the prop default to true OR remove it directly. Code needs to be cleaned up:

• Delete properties in BaseDropdown, GlDisclosureDropdown, and GlCollapsibleListbox
• Delete two wrappers used in BaseDropdown, listening to focusin by default (neither opt-on nor opt-out)
• Delete setting in config.js