BFormCheckbox

#2249 (closed) removed the usage from in BFormCheckbox GlFormCheckbox. The problem is that BFormCheckboxGroup does use BFormCheckbox if I'm not mistaken via mixins/form-radio-check-group.js, so I think we need to migrate GlFormCheckboxGroup first. Afterwards, we can remove the traces:

• Remove packages/gitlab-ui/src/vendor/bootstrap-vue/src/components/form-checkbox/form-checkbox.js
• Remove packages/gitlab-ui/src/vendor/bootstrap-vue/src/components/form-checkbox/form-checkbox.spec.js
• Remove entry from packages/gitlab-ui/src/vendor/bootstrap-vue/src/components/form-checkbox/index.js
• Remove entry from packages/gitlab-ui/src/vendor/bootstrap-vue/src/components/form-checkbox/package.json
• Remove BCheckbox specific docs from packages/gitlab-ui/src/vendor/bootstrap-vue/src/components/form-checkbox/README.md
• Remove imported consts, functions, mixins, etc. which might have been used in the now removed code