Remove extra assertions from end-to-end test

What does this MR do?

This MR removes extra assertions from an end-to-end test to avoid possible flakiness and speed things up a little bit.

By asserting that the epic title is the same of the issue that was promoted we are already testing that issue promotion works, so better to remove these extra assertions that might cause test flakiness since sometimes automatic discussions take longer to render.

Also, by removing an extra step + assertion, we make the test a bit faster.

Finally, this way the code gets a bit shorter.

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