Example: get rid of the backwards yaru.dart dependency

[jpnurmi]

The example must depend on yaru_colors from path to be able to use API available in the local checkout. It didn't do that because:

Running "flutter pub get" in example...
Resolving dependencies...
Because yaru_colors_example depends on yaru ^0.3.0 which depends on yaru_colors ^0.1.0, yaru_colors from hosted is required.
So, because yaru_colors_example depends on yaru_colors from path, version solving failed.
pub get failed

The yaru dependency is unnecessary so instead of hacking around it with overrides, just remove it. After all, the purpose of this example is to present the color palettes, not how the theme looks. ;)

[jpnurmi]

This should help #19 pass the CI.