Revert "Add ActiveRecord::Migration.no_transaction accessor to disable wrapping a migration in a transaction."

This reverts commit bb92911e1cb70ed1f990610eac0f11c3441f9b0c.
transactions can be turned off per Migration.

Closes #9483.

There are SQL Queries that can't run inside a transaction. Since
the Migrator used to wrap all Migrations inside a transaction there
was no way to run these queries within a migration.

This patch adds `self.disable_ddl_transaction!` to the migration to
turn transactions off when necessary.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/migration.rb
	activerecord/test/cases/migration/logger_test.rb
	activerecord/test/cases/migration_test.rb
	guides/source/migrations.md
Merge pull request #13 from tenderlove/migration-txns

Backport disabling transactions in migrations