The TransformHandler class has been copied into MediaWiki core, along with the ParsoidHandler base class. They should not be deteted from the parsoid extension, to avoid divergance and duplication of effort.
NOTE: the transform endpoint must for now still be exposed by the parsoid extension. But the path should be handled by the class implemented in MW core.
NOTE: the PageHandler class cannot be removed from the parsoid extension at this time, since it is structurally different from the equivalent endpoint in MW core.