Remove mention of adb from instructions (flutter#4499)

* Remove mention of adb from instructions

This isn't a step we have our users do, so best that we don't do it either.

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -60,8 +60,6 @@ solutions = [
  * `cd src/flutter` (Change to the `flutter` directory of the `src` directory that `gclient sync` created in your `engine` directory.)
  * `git remote add upstream [email protected]:flutter/engine.git` (So that you fetch from the master `flutter/engine` repository, not your clone, when running `git fetch` et al.)
  * `cd ..` (Return to the `src` directory that `gclient sync` created in your `engine` directory.)
- * Add `.../engine/src/third_party/android_tools/sdk/platform-tools` to your path so that you can run the `adb` tool more easily. This is also required by the `flutter` tool, which is used to run Flutter apps.
- * Make sure you are still in the `src` directory that the `gclient sync` step created earlier.
  * If you're on Linux, run `sudo ./build/install-build-deps-android.sh`
  * If you're on Linux, run `sudo ./build/install-build-deps.sh`
  * If you're on Mac, install Oracle's Java JDK, version 1.7 or later.