Remove rails dependent: :destroy statement

Enforced by the database, and no callbacks need to be called.

Combined with 7b9b2c6099 and 294a8b8a6d this resolves
gitlab-org/gitlab-ce#31799

app/models/project.rb

@@ -175,7 +175,7 @@ class Project < ActiveRecord::Base
   has_many :builds, class_name: 'Ci::Build' # the builds are created from the commit_statuses
   has_many :runner_projects, dependent: :destroy, class_name: 'Ci::RunnerProject'
   has_many :runners, through: :runner_projects, source: :runner, class_name: 'Ci::Runner'
-  has_many :variables, dependent: :destroy, class_name: 'Ci::Variable'
+  has_many :variables, class_name: 'Ci::Variable'
   has_many :triggers, dependent: :destroy, class_name: 'Ci::Trigger'
   has_many :environments, dependent: :destroy
   has_many :deployments, dependent: :destroy

changelogs/unreleased/zj-clean-up-ci-variables-table.yml

+---
+title: Cleanup ci_variables schema and table
+merge_request:
+author:

db/migrate/20170508190732_add_foreign_key_to_ci_variables.rb

@@ -13,7 +13,6 @@ class AddForeignKeyToCiVariables < ActiveRecord::Migration
         FROM projects
         WHERE projects.id = ci_variables.project_id
       )
-      OR ci_variables.project_id IS NULL
     SQL
     add_concurrent_foreign_key(:ci_variables, :projects, column: :project_id)

spec/factories/ci/variables.rb

@@ -2,5 +2,7 @@ FactoryGirl.define do
   factory :ci_variable, class: Ci::Variable do
     sequence(:key) { |n| "VARIABLE_#{n}" }
     value 'VARIABLE_VALUE'
+
+    project factory: :empty_project
   end
 end

spec/models/ci/variable_spec.rb

 require 'spec_helper'
 describe Ci::Variable, models: true do
-  subject { Ci::Variable.new }
+  subject { build(:ci_variable) }
   let(:secret_value) { 'secret' }