refactor(GlFormInputGroup): move BSV scss into `GlFormInputGroup`

What does this MR do?

Closes #2875 (closed)

In !5028 (merged) we removed Bootstrap Vue components from GlFormInputGroup. In !5034 (merged) we removed the BInputGroup component from the src/vendor/bootstrap-vue directory but left the scss. This MR moves the scss out of the src/vendor/bootstrap-vue directory and next to the GlFormInputGroup component.

At first I was going to change the CSS classes to have gl- prefixes but after searching through the gitlab-org/gitlab directory I found some references to the Bootstrap classes so I was worried this could break things. Because of this I opted to keep the class names the same.

Screenshots or screen recordings

From gitlab!186749 (closed)

Before	After

Integration merge requests

• GitLab: gitlab!186749 (closed)

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