Remove legacy code from EE version of pipeline class

We used to have pipeline source feature only in EE, but we have ported
it to the CE, and now PIPELINE_SOURCE variable also exists in the CE, so
there is no need of the override anymore.

ee/app/models/ee/ci/pipeline.rb

@@ -6,13 +6,6 @@ module EE
         size_limit_exceeded: 21
       }.freeze
 
-      def predefined_variables
-        result = super
-        result << { key: 'CI_PIPELINE_SOURCE', value: source.to_s, public: true }
-
-        result
-      end
-
       def codeclimate_artifact
         artifacts.codequality.find(&:has_codeclimate_json?)
       end