Remove quickstart docs.

docs/quickstart.md

-Skia Quickstart Guide
-=====================
-
-
-WARNING: Several steps in this guide are out of sync with our automatically-
-tested, officially-supported processes for checking out and building Skia.
-The officially supported processes are the ones documented in https://skia.org;
-see https://skia.org/user/quick.
-
-The steps documented within this file are more experimental in nature.
-
-
-This guide assumes you've got `git`, `ninja`, and `python` on your path.
-
-1. First, checkout Skia:
-    * `git clone https://skia.googlesource.com/skia.git`
-    * `cd skia`
-2. Then download the dependencies.  You only need to rerun this when
-   the dependencies change.
-    * `python tools/git-sync-deps`
-3. Create our Ninja build files from our Gyp meta-build files.  You only need
-   to rerun this when you sync or change a `.gyp` file.
-    * `GYP_GENERATORS=ninja ./gyp_skia`
-4. Now, let's build Skia.  There are a few options:
-    * `ninja -C out/Debug`: no optimization, asserts enabled
-    * `ninja -C out/Release`: optimization, asserts disabled
-    * `ninja -C out/Coverage`: no optimization, asserts enabled, code coverage generated
-5. Run some tests:
-    * `out/Debug/dm`: runs golden master tests from gm/, unit tests from tests/
-6. Make some changes:
-    * `git checkout -b my-new-feature origin/master`
-    * `vim src/...`
-    * `git commit -am "Changes for my new feature."`
-    * `vim tests/...`
-    * `git commit --amend -a`
-    * `ninja -C out/Debug && out/Debug/dm && echo ok`
-7. Rebase your change onto the latest Skia code:
-    * `git pull --rebase`
-    * `ninja -C out/Debug && out/Debug/dm && echo ok`
-8. Upload your change and send it out for review:
-    * `git cl upload -r my-skia-reviewer@google.com -s`
-    * `git cl web`
-9. Go through code review, get an LGTM, submit using the checkbox on the code review page.