Remove unnecessary assertions from epic management end-to-end test

What does this MR do?

Closes #121573 (closed).

This MR removes two unnecessary assertions from an epic management end-to-end test.

The removed assertions sometimes cause this test to be flaky, due to the automatic discussions not being rendered in time, and so, it's better not to have them and have this test providing reliable feedback, than have extra assertions that cause flakiness.

In any case, we'll still have the assertions that the issue was added/removed to/from the epic, in the epic page.

Finally, this change saves us five lines of code.

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