Simplify relation between a build and metadata

This removes erroneously defined polymorphic association, because
specifying `belongs_to` relationship with a class that already supports
polymorphic associations works out-of-the-box.

app/models/ci/build_metadata.rb

@@ -10,9 +10,7 @@ module Ci
 
     self.table_name = 'ci_builds_metadata'
 
-    belongs_to :build, class_name: 'CommitStatus',
-                       polymorphic: true, # rubocop:disable Cop/PolymorphicAssociations
-                       inverse_of: :metadata
+    belongs_to :build, class_name: 'CommitStatus'
     belongs_to :project
 
     before_create :set_build_project