Remove external link styles from CSS-Library

[jamigibbs]

Description

With the va-link web component supporting external links, we should remove the old external link styles that were moved from Formation to the CSS-Library.

Tasks

• Check vets-website, content-build, and next-build for uses of the external link CSS classes .usa-external_link & .usa-external_link-alt and replace them with va-link if found.
• Remove the unused external link styles from CSS-Library
• Remove any related images assets:
  - external-link.png
  - external-link.svg
  - external-link-hover.png
  - external-link-hover.svg

Acceptance Criteria

• the external link CSS classes and assets are moved from the CSS-Library