refactor: Remove duplicate buttonSizeOptionsMap constant

What does this MR do?

We previously had two very similar constants for button sizes, we probably only need one.

1. Delete buttonSizeOptions
2. Rename buttonSizeOptionsMap to buttonSizeOptions (shorter name)
3. Use Object.keys(buttonSizeOptions) to get the small/medium values

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
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.