Now that user links is enabled everywhere, we can drop the associated feature flag and code. This will simplify the code and our ability to test, and reduce the amount of CSS/JS we ship to end users.
Risk: Be careful not to turn the user links into a drop down in legacy Vector.
# A/C
[] Drop VectorConsolidateUserLinks config flag.
[] Skin::shouldConsolidateUserLinks is removed. In some cases replace the code with `$this->isLegacy()`
[] Drop any unused CSS relating to supporting modern Vector without user links into legacy mode.
[] Drop references to `is-consolidated-user-links` template variable
[] Retain vector-search-box-collapses and vector-search-box-show-thumbnail classes.
# QA
- Make sure legacy Vector still shows the user links as a flat horizontal list of links
- Make sure modern Vector shows the dropdown