Remove `BButton` from `GlButton`

BootstrapVue props we should support

• active
• active-class
• block
• disabled
• exact-active-class
• href
• prefetch
• rel
• replace
• size
• target
• to
• type
• variant

BootstrapVue props we should consider not supporting

https://gitlab-org.gitlab.io/frontend/bootstrapvue-component-usage-breakdown/components.json can be used as a starting point to see if the prop is used, but should not be considered an exhaustive list (it does not check https://gitlab.com/gitlab-org/customers-gitlab-com/ or Switchboard, please do this manually). We should weigh the pros and cons of supporting each prop individually.

• append
• exact
• exact-path
• exact-path-active-class
• no-prefetch
• pill
• pressed
• router-component-name
• squared
• tag

BootstrapVue slots we should support

• default

BootstrapVue events we should support

• click