WIP: fix(button): Remove btn class from GlButton component

What does this MR do?

Removes the .btn class from the button component. This prevents that leaky styles from GitLab affect GlButton styles. The .btn class apply button styles from Bootstrap. GlButton does not need these styles because it is already styled based on Pajamas specs.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.