Hide search icon again on larger breakpoints

What does this MR do?

Fixes a problem where both search icon and search box where visible on larger breakpoints. We can move back to the new gl- classes once gitlab-ui!2246 (closed) is merged and the new gitlab-ui update is being used.

Screenshots (strongly suggested)

Before	After

How to test?

You may want to check this out with both the :combined_menu feature flag on and off.

Description	Feature.enable(:combined_menu)	Feature.disable(:combined_menu)
Large display - No search icon visible 🎉
Medium display - Search icon visible 🎉
Small display - Search icon not visible with feature flag on 🎉

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to #333381 (closed)