Remove btn-link styles from buttons.scss

Removes btn-link styles from buttons.scss. No visual changes.

btn-link elements within the IDE have their own styles and are not impacted by this change:

Other btn-link elements need the gl-button class to be added to them to inherit the btn-link styles defined in gitlab-ui, whoch are the same as the block we're removing from buttons.scss here.

Here are some of these elements, which don't get any visual changes:

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Related to #362686 (closed)