feat(GlIcon): remove support for deprecated icons sizes

mentioned in commit 3abb22c1

mentioned in merge request gitlab!73463 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2616 (merged)

mentioned in merge request status-page!401 (merged)

mentioned in merge request gitlab!73857 (merged)

mentioned in merge request gitlab-docs!2242 (merged)

mentioned in merge request gitlab-docs!2245 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2637 (closed)