Remove PImpl scaffolding from CUDA version of LINCS

Remove PImpl scaffolding from CUDA version of LINCS

The CUDA implementation of LINCS was initially introduced as a
stand-alone feature. This required hiding CUDA-specific variables
and subroutines into the private implementation subclass. Since the
LINCS is not a part of Update and Constraints module, this is no
longer required and can be removed.

Refs #2816, #2888

Change-Id: I9698224d4702dfb8d99106999335c62e83a511df