Simplify DTCL registration support classes

Historically, there was support for registration of both
data tree change listeners and data change listeners. Common
infrastructure interfaces, abstract and generic classes were introduced,
since registration of both listener types required similar steps.

Data change listeners were deprecated and removed so there is no need
for this common infrastructure anymore. Get rid of unnecessary
interfaces, abstract classes and type variables. Now they just add
complexity to the code.

Change-Id: Ie2ce510bb5b49b892bcf8e5c562c96ea8345418e
Signed-off-by: Jakub Morvay <jmorvay@frinx.io>