Remove markdown templates and their generation. (#3600)

* Remove generated templates for aot compiler.

* Remove more aot template tests.

* Remove generator template.

* Remove templates/md/

Author: kallentu

build.yaml

@@ -11,7 +11,6 @@ builders:
     build_extensions:
       ".dart":
         - ".aot_renderers_for_html.dart"
-        - ".aot_renderers_for_md.dart"
         - ".runtime_renderers.dart"
     build_to: "source"