CUDA: Do not create CMAKE_CUDA_HOST_COMPILER cache entry
Users can create it through an explicit command-line option if desired.
Initializing the variable as an empty cache entry can wipe out a normal
variable of the same name that may have been set by a toolchain file.
Since commit 489c52ce we save the value
CMAKE_CUDA_HOST_COMPILER persistently in the compiler information
file as a normal variable.
Fixes: #17935 (closed)