[ARGG-812]: Remove svg imports and icon functions

[olliecurtis]

As part of improving build times and moving away from importing svgs via Sass, we update mixins to the following:

• Remove imports from bpk-svgs
• Remove the bpk-icon mixin as we no longer support this and recommend people use svgs via the bpk-component-icon in JS

To be used in the release notes:

Migration guide

If previously you were using the following for selecting icons.

.selector {
   @include bpk-icon(flight, $bpk-color-sky-gray-tint-02, large);
}

You will now need to do the following in JS: https://github.com/Skyscanner/backpack/tree/main/packages/bpk-component-icon#basic-usage

Remember to include the following changes:

• README.md (If you have created a new component)
• Component README.md
• Tests
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[github-actions]

Visit https://backpack.github.io/storybook-prs/3048 to see this build running in a browser.

[xalechez]

LGTM
Let's wait a while if anyone else wants to review this one and merge/release it

[github-actions]

Visit https://backpack.github.io/storybook-prs/3048 to see this build running in a browser.

[github-actions]

Visit https://backpack.github.io/storybook-prs/3048 to see this build running in a browser.