Remove unnecessary class

What does this MR do and why?

gl-md-flex-nowrap has been defined in gitlab-ui. Therefore, we replace it for the for the standard gitlab-ui one.

The only place where the class is used is app/assets/javascripts/ci/ci_variable_list/components/ci_variable_modal.vue

Screenshots or screen recordings

Continues to work as expected:

Screen_Recording_2023-04-25_at_21.20.35

How to set up and validate locally

1. Go to https://gdk.test:3443/flightjs/Flight/-/settings/ci_cd
2. Expand the Variables section
3. Select Add variable button
4. On the Key field add a AWS variable (start with letter a)
5. Play with different screen sizes to see the effect of the wrapping.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.