"ngs" library is a leftover of a not implemented concept, this library
adds additional encapsulation between interface and final classes.
Right now it is hard to guess what functionality should be encapsulated
in ngs classes. Most problematic are: "ngs::Session" and "ngs::Client".

Remove "ngs::Client", and "ngs::Session" classes from X Plugin, code
from those classes merge into "xpl::Client", "xpl::Session".

RB: 25415
Reviewed-by: Lukasz Kotula <lukasz.kotula@oracle.com>