Remove feature flag for customize homepage banner [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

• Removes customize homepage feature flag.
• Moves some specs that could be cheaper view specs out of feature specs for dashboard.
• Utilizes let_it_be to reduce factory creation.

How to test locally(gdk)

• ensure homepage not already customized.
• log in and ensure you see the screenshot as below of the banner.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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

Related to #299895