Fix E2E test - wait for branches to not be present

What does this MR do?

When testing if branches are deleted, wait for the branch element to not be present. Do so by checking for the absence of the branch element, rather than checking for the presence of text and then negating the result (the latter is less efficient since it will wait the full timeout duration in the expected case). Do the same for both checks (second and third branches).

This should avoid flakiness when branches take a little too long to be deleted. It also makes the checks more specific and consistent.

The test is left in quarantine because we should confirm it passes on Staging before unquarantining it.

See https://gitlab.com/gitlab-org/quality/nightly/issues/62

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