Update Devtools README.

This drops the section about devtoolslib, which became an implementation
detail. mojo_* top-level tools are now *the* supported way of consuming
devtools.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1265243002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 9a988ce4d585654cf13d2ae9ee9a4bf844aff2ee

README.md

diff --git a/README.md b/README.md
index 5176b7e..eb7d5c6 100644
--- a/README.md
+++ b/README.md
@@ -128,14 +128,6 @@
 mojo_debug device stack
 ```
 
-### devtoolslib
-
-**devtoolslib** is a Python module containing the core scripting functionality
-for running Mojo apps: shell abstraction with implementations for Android and
-Linux and support for apptest frameworks. The executable scripts in devtools are
-based on this module. One can also choose to embed the functionality provided by
-**devtoolslib** in their own wrapper.
-
 ## Development
 
 The library is canonically developed [in the mojo