remove temporary metric
remove temporary metric

Note: This metric is always "None", and was only used temporarily to
prove that no `enrollment_action` was getting through.

ARCH-1253

edx/edx-platform,appsembler/edx-platform,eduNEXT/edunext-platform,appsembler/edx-platform,EDUlib/edx-platform,angelapper/edx-platform,appsembler/edx-platform,msegado/edx-platform,mitocw/edx-platform,eduNEXT/edx-platform,EDUlib/edx-platform,angelapper/edx-platform,edx-solutions/edx-platform,angelapper/edx-platform,edx-solutions/edx-platform,edx/edx-platform,msegado/edx-platform,stvstnfrd/edx-platform,appsembler/edx-platform,cpennington/edx-platform,stvstnfrd/edx-platform,EDUlib/edx-platform,stvstnfrd/edx-platform,eduNEXT/edx-platform,cpennington/edx-platform,EDUlib/edx-platform,arbrandes/edx-platform,edx-solutions/edx-platform,eduNEXT/edunext-platform,edx/edx-platform,edx/edx-platform,mitocw/edx-platform,msegado/edx-platform,eduNEXT/edunext-platform,msegado/edx-platform,mitocw/edx-platform,cpennington/edx-platform,msegado/edx-platform,eduNEXT/edunext-platform,stvstnfrd/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,edx-solutions/edx-platform,cpennington/edx-platform,arbrandes/edx-platform,arbrandes/edx-platform,mitocw/edx-platform,eduNEXT/edx-platform