Remove `GlNavItemDropdown`

Look through all projects that consume GitLab (see pinned message in #tg_bootstrap_vue slack channel) and make sure GlNavItemDropdown is not used. Remove if it is not used and mark as a breaking change.

• https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/switchboard/
• https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com
• https://gitlab.com/gitlab-org/customers-gitlab-com/
• https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo
• https://gitlab.com/gitlab-org/gitlab-docs/
• https://gitlab.com/gitlab-org/status-page
• https://gitlab.com/gitlab-org/frontend/pajamas-adoption-scanner
• https://gitlab.com/gitlab-org/frontend/playground/vue-compat-scanner
• https://gitlab-org.gitlab.io/frontend/playground/gitlab-code-scanner
• https://gitlab.com/gitlab-org/editor-extensions/gitlab-jetbrains-plugin
• https://gitlab.com/gitlab-org/gitlab-vscode-extension/