Remove validation of MR level approval rules in merge requests

What does this MR do?

This removes MergeRequest#validate_approval_rule_source which was responsible to check whether the MR level approval rules matches the existing project level rules.

Added the validation in ApprovalMergeRequestRule to check if the project of approval_project_rule matches the merge request's project. This way we can ensure that the source rule should still be in the same project.

This will make the logic simpler for the upcoming API to create MR level approval rules.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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

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

#12607 (closed)