[wip/talos-metapkg-ros-control-sot] Fix Makefile

Fix DISTFILES definition by removing inclusion of
meta-pkgs/ros-base/Makefile.common out of context. The package is hosted
@openrobots.org, no need to use common ROS definitions.

While here, drop a number of useless statements:
 - setting PKGBASE manually is useless
 - setting WRKSRC is useless for this package
 - setting CMAKE_PREFIX_PATH is useless (especially to an empty location,
   ROBOTPKG_BASE is an optional user setting and almost never needed).