Retire links to Interface document (#62)

docs/interface.md is a stub that redirects.

We should no longer direct people to it.

Author: ewilligers

README.md

@@ -7,13 +7,13 @@ data from other sites and apps.
 
 * [Explainer document](docs/explainer.md), a high-level overview of the proposal.
 * [Specification](https://wicg.github.io/web-share-target/).
-* [Interface document](docs/interface.md), an informal spec.
 * [Native integration survey](docs/native.md), for platform-specific matters.
 
 This is a product of the [Ballista
 project](https://github.com/chromium/ballista), which aims to explore
 website-to-website and website-to-native interoperability.
 
+
 ## Licensing and contributions
 
 See [LICENSE](LICENSE.md) and [CONTRIBUTING](CONTRIBUTING.md).

docs/explainer.md

@@ -17,7 +17,7 @@ project](https://github.com/chromium/ballista), which aims to explore
 website-to-website and website-to-native interoperability.
 
 See also:
-* [Interface document](interface.md), an informal spec.
+* [Specification](https://wicg.github.io/web-share-target/).
 * [Native integration survey](native.md), for platform-specific matters.
 
 ## User flow