Remove unneeded monkey-patch

Changes all calls to data_source_exists? to table_exists?
since that is the intent of these calls

config/initializers/0_license.rb

@@ -10,7 +10,7 @@ Gitlab.ee do
   end
 
   # Needed to run migration
-  if ActiveRecord::Base.connected? && ActiveRecord::Base.connection.data_source_exists?('licenses')
+  if ActiveRecord::Base.connected? && ActiveRecord::Base.connection.table_exists?('licenses')
     message = LicenseHelper.license_message(signed_in: true, is_admin: true, in_html: false)
     if ::License.block_changes? && message.present?
       warn "WARNING: #{message}"

config/initializers/active_record_migration.rb

-require 'active_record/migration'
-
-module ActiveRecord
-  class Migration
-    # data_source_exists? is not available in 4.2.10, table_exists deprecated in 5.0
-    def table_exists?(table_name)
-      ActiveRecord::Base.connection.data_source_exists?(table_name)
-    end
-  end
-end

config/initializers/load_balancing.rb

@@ -3,7 +3,7 @@
 # We need to run this initializer after migrations are done so it doesn't fail on CI
 
 Gitlab.ee do
-  if ActiveRecord::Base.connected? && ActiveRecord::Base.connection.data_source_exists?('licenses')
+  if ActiveRecord::Base.connected? && ActiveRecord::Base.connection.table_exists?('licenses')
     if Gitlab::Database::LoadBalancing.enable?
       Gitlab::Database.disable_prepared_statements
 

lib/gitlab/background_migration/prepare_untracked_uploads.rb

@@ -55,7 +55,7 @@ module Gitlab
       def ensure_temporary_tracking_table_exists
         table_name = :untracked_files_for_uploads
 
-        unless ActiveRecord::Base.connection.data_source_exists?(table_name)
+        unless ActiveRecord::Base.connection.table_exists?(table_name)
           UntrackedFile.connection.create_table table_name do |t|
             t.string :path, limit: 600, null: false
             t.index :path, unique: true

lib/tasks/gitlab/db.rake

@@ -33,7 +33,7 @@ namespace :gitlab do
       # Removes the entry from the array
       tables.delete 'schema_migrations'
       # Truncate schema_migrations to ensure migrations re-run
-      connection.execute('TRUNCATE schema_migrations') if connection.data_source_exists? 'schema_migrations'
+      connection.execute('TRUNCATE schema_migrations') if connection.table_exists? 'schema_migrations'
 
       # Drop tables with cascade to avoid dependent table errors
       # PG: http://www.postgresql.org/docs/current/static/ddl-depend.html

scripts/insert-rspec-profiling-data

@@ -14,10 +14,6 @@ module RspecProfiling
         Result.establish_connection(results_url)
       end
 
-      def prepared?
-        connection.data_source_exists?(table)
-      end
-
       def results_url
         ENV['RSPEC_PROFILING_POSTGRES_URL']
       end

spec/lib/gitlab/background_migration/populate_untracked_uploads_spec.rb

@@ -114,7 +114,7 @@ describe Gitlab::BackgroundMigration::PopulateUntrackedUploads, :sidekiq, :migra
     it 'does not drop the temporary tracking table after processing the batch, if there are still untracked rows' do
       subject.perform(1, untracked_files_for_uploads.last.id - 1)
 
-      expect(ActiveRecord::Base.connection.data_source_exists?(:untracked_files_for_uploads)).to be_truthy
+      expect(ActiveRecord::Base.connection.table_exists?(:untracked_files_for_uploads)).to be_truthy
     end
 
     it 'drops the temporary tracking table after processing the batch, if there are no untracked rows left' do