[Extensions] Remove reference to .gyp files, removed years ago.

.gyp files were removed years ago for performance reasons.

Bug: None
Change-Id: I01fded482a7cde8dd0732e0724ff54316afebb61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323961
Commit-Queue: Solomon Kinard <solomonkinard@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792410}

chrome/browser/extensions/api/README.txt

@@ -19,11 +19,7 @@ Create "chrome/common/extensions/api/experimental_foo.json". For inspiration
 look at the "app" API. Include descriptions fields to generate the
 documentation.
 
-2) Add your API specification to api.gyp.
-Add "experimental_foo.json" to the "schema_files" section in
-"chrome/common/extensions/api/api.gyp".
-
-3) Write the API function handlers.
+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.
@@ -36,11 +32,7 @@ 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) Add your API specification to api.gyp.
-Add "experimental_foo.idl" to the "schema_files" section in
-"chrome/common/extensions/api/api.gyp".
-
-3) Write the API function handlers.
+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.
@@ -48,27 +40,27 @@ how to do that.
 --------------------------------------------------------------------------------
 STEPS COMMON TO BOTH APPROACHES
 
-6) Write support classes for your API
+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.
 
-7) Update the project with your new files.
+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
 
-8) Add a stub template in ../docs/templates/public corresponding to your API.
+5) Add a stub template in ../docs/templates/public corresponding to your API.
 See other templates for inspiration.
 
-9) Run ../docs/templates/server2/preview.py to view the generated documentation.
+6) Run ../docs/templates/server2/preview.py to view the generated documentation.
 
 --------------------------------------------------------------------------------
 WRITING TESTS
 
-12) Write a unit test for your API.
+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".