Drop the sky config file and deprecated instructions in README.

mojo/tools/configs/sky

-# This is a configuration file one can use to run Sky apps from a local Sky
-# checkout using a locally built Mojo shell. As Sky is a layer on top of Mojo,
-# this file does not really belong in the Mojo repo and could hopefully move to
-# sky_engine.
-
-# To use this you need to check out sky_engine
-# (https://github.com/domokit/sky_engine ) and build for Android.
-
-# Then, run the lengthy command-line:
-#
-#  mojo/devtools/common/mojo_run \
-#    --android \
-#    --config-file mojo/tools/configs/sky \
-#    --config-alias SKY_SRC=/path_to_your_checkout/sky_engine/src \
-#    --embed https://sky/examples/hello_world/lib/main.dart
-
-# The sky apps are served from the Sky checkout, so all of
-# sky/packages/sky/example is available under https://sky/examples .
-
-{
-  'dev_servers': [
-    {
-      'host': 'https://sky/',
-      'mappings': [
-        ('packages/', [
-          '@{SKY_SRC}/sky/packages/workbench/packages'
-        ]),
-        ('examples/', [
-          '@{SKY_SRC}/examples'
-        ]),
-        ('', [
-          '@{SKY_SRC}/out/android_Debug',
-        ]),
-      ],
-    },
-    {
-      'host': 'https://core.mojoapps.io/',
-      'mappings': [
-        ('', ['@{BUILD_DIR}']),
-      ],
-    },
-  ],
-
-  'content_handlers': {
-    'application/dart': 'https://sky/sky_viewer.mojo',
-  }
-}