Remove outdated HACKING.md document

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sky/HACKING.md

-Hacking on Sky
-==============
-
-Building
---------
-
-* Follow the setup & build instructions for [Mojo](https://github.com/domokit/mojo)
-
-The build directory will be `out/Debug` for Linux debug builds, and
-`out/Release` for Linux release builds. For Android builds, prefix
-`android_`, as in, `android_Debug`.
-
-For Sky on iOS, you can try following these [experimental instructions](https://docs.google.com/document/d/1qm8Vvyz8Mngw6EsSg4FELQ7FOegjBJ5Itg9OBJGN5JM/edit#heading=h.ikz9bdwswdct).
-
-Running applications
---------------------
-
-To run an application on your device, run:
-
-* `mojo/devtools/common/mojo_shell --sky [url] --android`
-
-When the shell is running, `mojo/devtools/common/debugger` allows you to
-collect traces, symbolize stack crashes and attach gdb if needed. Refer to the
-[documentation](https://github.com/domokit/mojo#debugging-tracing-profiling)
-for the details.
-
-Running tests
--------------
-
-Tests are only supported on Linux currently.
-
-* ``sky/tools/test_sky --debug``
-  * This runs the tests against ``//out/Debug``. If you want to run against
-    ``//out/Release``, omit the ``--debug`` flag.
-
-Running tests manually
-----------------------
-
-Running tests manually lets you more quickly iterate during
-development; rather than having to compile and rerun all the tests,
-then trawl through the build output to find the current results, you
-can just run the test you're working on and reload it over and over,
-seeing the output right there on your console.
-
-* ``sky/tools/skygo/linux64/sky_server -v -p 8000 out/Debug out/Debug/gen/dart-pkg/packages``
-* ``out/Debug/mojo_shell --args-for="mojo:native_viewport_service --use-headless-config --use-osmesa" --args-for"=mojo:sky_viewer --testing" --content-handlers=application/dart,mojo:sky_viewer --url-mappings=mojo:window_manager=mojo:sky_tester,mojo:surfaces_service=mojo:fake_surfaces_service mojo:window_manager``
-* The ``sky_tester`` should print ``#READY`` when ready
-* Type the URL you wish to run, for example ``http://127.0.0.1:8000/sky/tests/widgets/dialog.dart``, and press the enter key
-* The harness should print the results of the test.  You can then type another URL.
-
-Writing tests
--------------
-
-We recommend using the [Dart
-``test``](https://pub.dartlang.org/packages/test) testing framework.
-See [``sky/tests/raw/color_bounds.dart``](tests/raw/color_bounds.dart)
-for an example.
-
-Debugging Sky
--------------
-
-This document aims to explain how to debug Sky itself.
-
-### C++
-
-Launch a debug Sky build on Android as usual:
-
-```
-mojo/devtools/common/mojo_shell --sky [url] --android`
-```
-
-and use the debugger to attach gdb:
-```
-mojo/devtools/common/debugger gdb attach
-```
-
-Once gdb has loaded, hit `c` to continue the execution. When your app crashes,
-it will pause in the debugger. At that point, regular gdb commands will work:
-`n` to step over the current statement, `s` to step into the current statement,
-`f` to step out of the current block, `c` to continue until the next breakpoint
-or exception.
-
-### Dart
-
-Use Observatory.