Remove REST call and related logic for error details page

What does this MR do?

Before this MR, REST and GraphQL calls coexisted on error detail page, cause GraphQL endpoint didn't have all required data to render it. Now after missing fields were added on BE side (done in different MR), FE can get all data via graphql endpoint and doesn't need to do REST call anymore. So it was removed.

BE changes here are only related to removal of issue-details-path - it is REST endpoint path. And as FE doesn't need it anymore, it is removed.

FE changes have removal of REST call, related vuex store actions, mutations, update of the tests. All data is now coming from GraphQL endpoint

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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

References #197137 (closed)