Remove pubspec.yamls for sky/sdk/example

As long as these examples are part of sky/sdk, they don't need pubspec.yamls
because they inherit them from sky/sdk/pubspec.yaml. Eventually we'll want to
move these examples to their own repo, but we're not ready for that yet.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1233453005 .

sdk/example/fitness/pubspec.yaml

-name: fitness
-author: Chromium Authors <sky-dev@googlegroups.com>
-description: A demo application using Sky that tracks fitness data
-homepage: https://github.com/domokit/sky_sdk/tree/master/sdk/lib/example/stocks
-version: 0.0.1
-dependencies:
-  sky: '>=0.0.10 <1.0.0'

sdk/example/game/pubspec.yaml

-name: game
-dependencies:
-  sky: '>=0.0.10 <1.0.0'

sdk/example/hello_world/pubspec.yaml

-name: hello_world
-author: Chromium Authors <sky-dev@googlegroups.com>
-description: Hello, world using Sky
-homepage: https://github.com/domokit/sky_sdk/tree/master/packages/sky/example/hello_world
-version: 0.0.1
-dependencies:
-  sky: '>=0.0.10 <1.0.0'

sdk/example/mine_digger/pubspec.yaml

-name: mine_digger
-author: Chromium Authors <sky-dev@googlegroups.com>
-description: Classic minesweeper-inspired game
-homepage: https://github.com/domokit/sky_sdk/tree/master/packages/sky/example/mine_digger
-version: 0.0.1
-dependencies:
-  sky: '>=0.0.10 <1.0.0'

sdk/example/raw/pubspec.yaml

-name: raw
-dependencies:
- sky: any

sdk/example/stocks/pubspec.yaml

-name: stocks
-author: Chromium Authors <sky-dev@googlegroups.com>
-description: A demo application using Sky that shows stock data
-homepage: https://github.com/domokit/sky_sdk/tree/master/sdk/lib/example/stocks
-version: 0.0.1
-dependencies:
-  sky: '>=0.0.10 <1.0.0'