refactor(ngTransclude): use transclusion function passed in to link

Since we now pass in the transclusion function directly to the link function, we no longer need
the old scheme whereby we saved the transclude function injected into the controller for later
use in during linking.

Additionally, this change may aid in correcting a memory leak of detached DOM nodes (see #6181
for details).

This commit removes the controller and simplifies ngTransclude.

Closes #5375
Closes #6181