Remove obsolete catch block in getLayerAs

[tobrun]

Closes #12473, removed catch clause as noted in the issue:

(T) is reduced to (Layer) at Runtime and it just makes the compiler happy, but changes nothing. Therefor the catch block will never run. This method removes some need for the callers to cast, but it will not protect them from CastClassExceptions.

The cast class exception is expected, provides same developer experience as with similar constructs in other libraries as for example ButterKnife.

[LukasPaczos]

Javadoc for the method should be updated as well.

[tobrun]

#13091 (review) was addressed in 7b4b4bb