Drop default rule_type when creating project approval rules

Project approval rules already have a default column `rule_type` set &
shouldn't require an explicit one.

This actually interferes with `ApprovalRules::CreateService` which
assumes an explicitly passed `rule_type` should be set by using a
`#reverse_merge`.  By dropping the forced default we can roll out a more
gradual frontend update to trigger this behavior rather than require
changes on both the backend and frontend

ee/lib/api/project_approval_rules.rb

@@ -29,7 +29,7 @@ module API
           params do
             requires :name, type: String, desc: 'The name of the approval rule'
             requires :approvals_required, type: Integer, desc: 'The number of required approvals for this rule'
-            optional :rule_type, type: String, desc: 'The type of approval rule', default: 'regular'
+            optional :rule_type, type: String, desc: 'The type of approval rule'
             optional :users, as: :user_ids, type: Array, coerce_with: ARRAY_COERCION_LAMBDA, desc: 'The user ids for this rule'
             optional :groups, as: :group_ids, type: Array, coerce_with: ARRAY_COERCION_LAMBDA, desc: 'The group ids for this rule'
           end

ee/spec/requests/api/project_approval_rules_spec.rb

@@ -133,6 +133,14 @@ describe API::ProjectApprovalRules do
 
         expect(json_response.symbolize_keys).to include(params)
       end
+
+      it 'sets rule_type as report_approver if name matches default name for security reports' do
+        expect do
+          post api(url, current_user), params: params.merge(name: ApprovalProjectRule::DEFAULT_NAME_FOR_SECURITY_REPORT)
+        end.to change { ApprovalProjectRule.report_approver.count }.from(0).to(1)
+
+        expect(response).to have_gitlab_http_status(201)
+      end
     end
   end