Remove status checks from approval rules and update feature specs

What does this MR do?

With #327638 (closed) we have moved status checks from MR approval rules to their own area in the projects MR settings. This MR removes all the status checks related code (added in !57158 (merged) and !57528 (merged)) from MR approval rules and updates the feature specs so we know the new status checks area is covered.

Screenshots (strongly suggested)

MR approval rules	MR status checks
Screen_Recording_2021-06-08_at_13.00.31	Screen_Recording_2021-06-08_at_13.02.21

Setup & testing

Firstly enable the feature flag :ff_compliance_approval_gates

MR approval rules

1. Go to [HOST]/gitlab-org/gitlab-test/edit and expand the Merge request (MR) approvals section
2. Click the Add approval rule button and try breaking the values in the form
3. Add a new approval rule
4. Check that the table now contains the new approval rule
5. Edit the approval rules approvals required number
6. Click the approval rules edit button and try breaking the values in the form
7. Update the approval rule
8. Check that the table now contains the updated approval rule
9. Delete the approval rule

Status checks

1. Go to [HOST]/gitlab-org/gitlab-test/edit and expand the Merge requests section
2. Click the status checks add button and try breaking the values in the form
3. Add a new status check
4. Check that the table now contains the new status check
5. Click the status checks edit button and try breaking the values in the form
6. Update the status check
7. Check that the table now contains the updated status check
8. Delete the status check

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
  • User-facing changes are behind the feature flag :ff_compliance_approval_gates
• [-] 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.

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

Related to #332017 (closed)