refactor: remove unused assets, move remaining assets out of payload packages (#12874)

This PR removes the `packages/payload/src/assets` folder for the
following reasons:
- they were published to npm. Removing this decreases the install size
of payload (excluding dependencies) from 6.22MB => 5.12MB
- most assets were unused. The only used ones were moved to a different
directory that does not get published to npm

This also updates some outdated asset URLs in our examples

examples/auth/src/app/(app)/_components/Header/index.tsx

@@ -14,12 +14,12 @@ export const Header = () => {
           <picture>
             <source
               media="(prefers-color-scheme: dark)"
-              srcSet="https://raw.githubusercontent.com/payloadcms/payload/master/src/admin/assets/images/payload-logo-light.svg"
+              srcSet="https://raw.githubusercontent.com/payloadcms/payload/main/packages/ui/src/assets/payload-logo-light.svg"
             />
             <Image
               alt="Payload Logo"
               height={30}
-              src="https://raw.githubusercontent.com/payloadcms/payload/master/src/admin/assets/images/payload-logo-dark.svg"
+              src="https://raw.githubusercontent.com/payloadcms/payload/main/packages/ui/src/assets/payload-logo-dark.svg"
               width={150}
             />
           </picture>