[autotest] Drop RPC support for specifying a test kernel.

Our legacy Autotest code included support for specifying a specific
kernel to be tested when creating a job.  That support wasn't
appropriate for Chrome OS, and didn't work.

All existing user level interfaces for specifying a kernel (CLI and
AFE) have been changed to drop this feature.  This change removes
support for the feature in the RPC calls.  Lower-level layers still
contain support that is unused, which will be removed in subsequent
CLs.

BUG=chromium:266704
TEST=Look at control file from AFE job creation page; run atest job create

Change-Id: Ifb0c6888f550f6cda98195b489ba8076cdf47e6b
Reviewed-on: https://chromium-review.googlesource.com/346570
Commit-Ready: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@google.com>
diff --git a/frontend/afe/rpc_utils.py b/frontend/afe/rpc_utils.py
index eea15b9..4b3e383 100644
--- a/frontend/afe/rpc_utils.py
+++ b/frontend/afe/rpc_utils.py
@@ -285,8 +285,7 @@
     return False
 
 
-def prepare_generate_control_file(tests, kernel, label, profilers,
-                                  db_tests=True):
+def prepare_generate_control_file(tests, profilers, db_tests=True):
     if db_tests:
         test_objects = [models.Test.smart_get(test) for test in tests]
     else:
@@ -309,8 +308,6 @@
         synch_count = max(test.sync_count for test in test_objects)
     else:
         synch_count = 1
-    if label:
-        label = models.Label.smart_get(label)
 
     if db_tests:
         dependencies = set(label.name for label
@@ -321,7 +318,7 @@
 
     cf_info = dict(is_server=is_server, synch_count=synch_count,
                    dependencies=list(dependencies))
-    return cf_info, test_objects, profiler_objects, label
+    return cf_info, test_objects, profiler_objects
 
 
 def check_job_dependencies(host_objects, job_dependencies):