[Extensions Docs] Remove old API documentation

The old docs at chrome/browser/extensions/api/README.txt are obsolete
with chrome/common/extensions/api/schemas.md and
extensions/docs/writing_a_new_api.md. Remove it.

Bug: None
Change-Id: If5de57295ad326b55742536435f4aa7dc524fac8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340165Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795738}

chrome/browser/extensions/api/README.txt

-This file describes steps and files needed when adding a new API to Chrome.
-Before you start coding your new API, though, make sure you follow the process
-described at:
-  http://www.chromium.org/developers/design-documents/extensions/proposed-changes/apis-under-development
-
-Two approaches are available for writing your API specification. The original
-approach relies on JSON specification files. The more recent and simpler system 
-uses Web IDL files, but does not yet support all the features of the JSON files.
-Discuss with a member of the extensions team (aa@chromium.org) before you decide
-which approach is better suited to your API.
-
-The following steps suppose you're writing an experimental API called "Foo".
-
---------------------------------------------------------------------------------
-APPROACH 1: JSON FILES
-
-1) Write your API specification.
-Create "chrome/common/extensions/api/experimental_foo.json". For inspiration 
-look at the "app" API. Include descriptions fields to generate the
-documentation.
-
-2) Write the API function handlers.
-Create foo_api.cc and foo_api.h under "chrome/browser/extensions/api/foo". You
-should use the JSON Schema Compiler. Look at the "permissions_api.cc" for
-details on how to do that.
-
---------------------------------------------------------------------------------
-APPROACH 2: IDL FILES
-
-1) Write your API specification.
-Create "chrome/common/extensions/api/experimental_foo.idl". For inspiration look
-at "alarms.idl". Include comments, they will be used to automatically generate
-the documentation.
-
-2) Write the API function handlers.
-Create foo_api.cc and foo_api.h under "chrome/browser/extensions/api/foo". You
-should use the JSON Schema Compiler. Look at the "alarms_api.cc" for details on
-how to do that.
-
---------------------------------------------------------------------------------
-STEPS COMMON TO BOTH APPROACHES
-
-3) Write support classes for your API
-If your API needs any support classes add them to
-"chrome/browser/extensions/api/foo". Some old APIs added their support classes
-directly to chrome/browser/extensions. Don't do that.
-
-4) Update the project with your new files.
-The files you created in (3) and (5) should be added to
-"chrome/chrome_browser_extensions.gypi".
-
---------------------------------------------------------------------------------
-GENERATING DOCUMENTATION
-
-5) Add a stub template in ../docs/templates/public corresponding to your API.
-See other templates for inspiration.
-
-6) Run ../docs/templates/server2/preview.py to view the generated documentation.
-
---------------------------------------------------------------------------------
-WRITING TESTS
-
-7) Write a unit test for your API.
-Create "chrome/browser/extensions/api/foo/foo_api_unittest.cc" and test each of
-your API methods. See "alarms_api_unittest.cc" for details. Once done add your
-.cc to "chrome/chrome_tests.gypi".