Revert "DxBuilder: Fixed discriminators for default value adapter lookup."

This reverts commit bf45f133656fb925e47d8a45295cae6522dbf39f.
The reverted commit was based on a wrong assumption: The adapted context
for default value adapters is actually *supposed* to be the container
for add forms, therefore the previous behavior of ftw.builder was correct.

Conflicts:

	docs/HISTORY.txt