Fix mysql specs

ee/app/models/ee/epic.rb

@@ -169,7 +169,7 @@ module EE
       # epic2 - parent: epic1
       # Returns: 2
       def deepest_relationship_level
-        return nil unless ::Gitlab::Database.postgresql?
+        return unless ::Gitlab::Database.postgresql?
 
         result =
           ActiveRecord::Base.connection.execute(

ee/spec/lib/gitlab/usage_data_spec.rb

@@ -54,7 +54,6 @@ describe Gitlab::UsageData do
       expect(count_data.keys).to include(*%i(
         projects_mirrored_with_pipelines_enabled
         epics
-        epics_deepest_relationship_level
         geo_nodes
         ldap_group_links
         ldap_keys

ee/spec/models/epic_spec.rb

@@ -443,7 +443,7 @@ describe Epic do
     end
   end
 
-  describe '.deepest_relationship_level' do
+  describe '.deepest_relationship_level', :postgresql do
     it 'returns the deepest relationship level between epics' do
       group_1 = create(:group)
       group_2 = create(:group)