Remove the code in ThreadableLoader for delegating ref/deref to subclasses

This logic was meaningful when we were inheriting multiple superclasses
with ref/deref defined. Currently they're unnecessary and just
complicating the source. Remove it.

BUG=none

Review URL: https://codereview.chromium.org/267703002

git-svn-id: svn://svn.chromium.org/blink/trunk@173102 bbb929c8-8fbe-4397-9dbb-9b2b20218538