CUDA: Drop unused/broken platform link flags placeholder
The <CMAKE_CUDA_LINK_FLAGS>
placeholder in CUDA rule variables comes
from the <CMAKE_CXX_LINK_FLAGS>
placeholder from which the CUDA rule
variables were originally derived. It is not a public interface for
adding link flags so no projects should be using it. It is needed for
platform information modules to specify platform-specific link flags for
the language. None of our platform modules set it, so it is unused.
Furthermore, it is broken as currently implemented. Some of the
contexts in which it is used need -Xlinker
and some do not.
Therefore it is not possible to use the placeholder at all.
Simply remove it for now. If some need for platform-specific CUDA link flags arises a new solution will be needed.