[generator_script] Remove support for allow_subannotations

I don't find any scripts is specifying this field from code search.
Hence, remove the support.

R=iannucci

Bug: 1017831
Change-Id: Id6cb590fd6846f948461310033a38cd3d05b43c1
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/2274729
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>
diff --git a/README.recipes.md b/README.recipes.md
index f824eeb..614d9ae 100644
--- a/README.recipes.md
+++ b/README.recipes.md
@@ -1704,9 +1704,6 @@
         status page. Note that these are write-only: The only way to read
         them is via the status page. There is intentionally no mechanism to
         read them back from inside of the recipes.
-  * allow_subannotations: allow this step to emit legacy buildbot
-    subannotations. If you don't know what this is, you shouldn't use it. If
-    you know what it is, you also shouldn't use it.
 ### *recipe_modules* / [isolated](/recipe_modules/isolated)
 
 [DEPS](/recipe_modules/isolated/__init__.py#1): [cipd](#recipe_modules-cipd), [context](#recipe_modules-context), [json](#recipe_modules-json), [path](#recipe_modules-path), [properties](#recipe_modules-properties), [raw\_io](#recipe_modules-raw_io), [runtime](#recipe_modules-runtime), [step](#recipe_modules-step)
diff --git a/recipe_modules/generator_script/api.py b/recipe_modules/generator_script/api.py
index ba8a531..5cb0119 100644
--- a/recipe_modules/generator_script/api.py
+++ b/recipe_modules/generator_script/api.py
@@ -16,7 +16,7 @@
 class GeneratorScriptApi(recipe_api.RecipeApi):
   ALLOWED_KEYS = frozenset([
     # supported by step module
-    'name', 'cmd', 'env', 'cwd', 'allow_subannotations',
+    'name', 'cmd', 'env', 'cwd',
 
     # implemented by GeneratorScriptApi
     'always_run', 'outputs_presentation_json',
@@ -75,9 +75,6 @@
             status page. Note that these are write-only: The only way to read
             them is via the status page. There is intentionally no mechanism to
             read them back from inside of the recipes.
-      * allow_subannotations: allow this step to emit legacy buildbot
-        subannotations. If you don't know what this is, you shouldn't use it. If
-        you know what it is, you also shouldn't use it.
     """
     f = '--output-json'
     step_name = 'gen step(%s)' % self.m.path.basename(path_to_script)
@@ -118,11 +115,7 @@
         try:
           cwd = self.m.path.abs_to_path(step['cwd']) if 'cwd' in step else None
           with self.m.context(env=step.get('env'), cwd=cwd):
-            self.m.step(
-              step['name'], cmd,
-              allow_subannotations=bool(step.get(
-                'allow_subannotations', False)),
-            )
+            self.m.step(step['name'], cmd)
         except self.m.step.StepFailure:
           failed_steps.append(step['name'])
         finally: