Remove migrate_drupal's MigratePluginManager implementation

Problem/Motivation

migrate_drupal provides an implementation of Migrate's MigratePluginManager, but as far as I can tell, its one overridden method is never called by anything. So the entire class is pointless, as is the overridden method in the base class.

Proposed Resolution

Remove the useless class and Migrate's unused plugin manager method.

API Changes

migrate_drupal no longer provides its own plugin manager; it uses Migrate's instead. The removed method is protected, so it shouldn't really affect anything.