Prune epics management end-to-end test

What does this MR do?

Prune epics management end-to-end test

This test fails intermittently on CI in the assertion after editing the epic title, as it can be seen in the below job:

https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/387863693

Editing and deletion of epics have already a good coverage in lower-level tests, as it can be seen from the following files:

• ee/spec/features/epics/update_epic_spec.rb
• ee/spec/features/epics/delete_epic_spec.rb

With that, it's safe to remove these steps from the end-to-end test suite and be sure that these functionalities are still being tested.

Besides that, now the test is focused on only one thing.

This MR also deletes a page object that was only used by the deleted test steps and it also deletes two page object methods that are not in use anymore.

Finally, this change reduces the code in 68 lines.

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