Clear Environment Search in CI/CD Variables

What does this MR do?

By clearing the search, it makes it easier for users to pick existing environments, as they are already listed, but filtered out by the current value.

Screenshots (strongly suggested)

Screen_Recording_2021-02-18_at_16.50.51

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done