Remove dashboard split button project select low usage[RUN-AS-IF-FOSS]

What does this MR do?

Closes #333147 (closed). This removes the dashboard split button (project selector) from these dashboard pages:

• http://localhost:3000/dashboard/issues
• http://localhost:3000/dashboard/merge_requests
• http://localhost:3000/dashboard/milestones
• http://localhost:3000/groups/flightjs/-/issues

Why?

This was initially kicked off by a thread in slack, further investigation followed: https://gitlab.slack.com/archives/C0AR2KW4B/p1627324666110200.

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).

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.

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team