Un-instrument Banzai::ReferenceExtractor

Instrumenting this class together with Gitlab::ReferenceExtractor causes
a StackError for some reason. Since Gitlab::ReferenceExtractor has most
of the interesting code we'll only instrument that class.

config/initializers/metrics.rb

@@ -85,9 +85,6 @@ if Gitlab::Metrics.enabled?
       config.instrument_instance_methods(const)
     end
 
-    config.instrument_methods(Banzai::ReferenceExtractor)
-    config.instrument_instance_methods(Banzai::ReferenceExtractor)
-
     config.instrument_methods(Banzai::Renderer)
     config.instrument_methods(Banzai::Querying)