Remove new_project_item_select split dropdown

This is a reused haml component being used on all our dashboard pages. It currently uses the now deprecated "default button selector" behavior (See epic for more details). Designs should be created, then tested to verify usability.

Update: We recently gathered data on usage of these split dropdown dashboard buttons. Turns out, only .02% of users use these buttons. Due to the lack of usage, this should be removed from the UI. Refactoring and putting time and effort into something that gets such little use may provide very little upside and increase usage only slightly. We constantly add to our UI, but it's also important to remove things to keep our UI clean from unused clutter. This is one of those rare cases.

Usage data

https://app.periscopedata.com/app/gitlab/901370/New-*-Button-Clicks