gn2bp: Skip generating pb2.py files

pb2.py files are not used by other targets.

Test: ./update_result.sh
Change-Id: If03d05180587460668810277c57bb11adb98d45b
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 8a33036..5ff17ff 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -886,7 +886,9 @@
         module.srcs.add(target.args[i+1])
         target.args[i + 1] = '$(location %s)' % target.args[i + 1]
       elif val == "--py-out-dir":
-        target.args[i + 1] = '$(genDir)/' + target.args[i + 1]
+        # pb2.py files are not used by others.
+        target.args[i] = ''
+        target.args[i + 1] = ''
 
   script = gn_utils.label_to_path(target.script)
   module.tool_files.add(script)