refactor(variables): Update icon size variables

What does this MR do?

• Updates default-icon-size from 18px to 16px. With the default icon grid being 16px, and 18px not even a part or our base-8 settings I think this can safely be updated.
• Removes the 18 option from $gl-icon-sizes. Interestingly, anywhere I could find .s18in GitLab’s CSS it explicitly set the size to18px` instead of relying on a variable.
• Removes the 10 option from $gl-icon-sizes. I couldn’t find .s10` being used anywhere.

On a related note, this is being updated on the product side in gitlab!53254 (merged).

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.

Security

N/A

Accessibility

N/A

Closes #1218 (closed)