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 v3.8.0-rc1~261^2~11 (CUDA: Use the host compiler for
linking CUDA executables and shared libs, 2016-09-19) we save the value
of `CMAKE_CUDA_HOST_COMPILER` persistently in the compiler information
file as a normal variable.

Fixes: #17935

Modules/CMakeDetermineCUDACompiler.cmake

@@ -40,7 +40,6 @@ else()
 endif()
 
 #Allow the user to specify a host compiler
-set(CMAKE_CUDA_HOST_COMPILER "" CACHE FILEPATH "Host compiler to be used by nvcc")
 if(NOT $ENV{CUDAHOSTCXX} STREQUAL "")
   get_filename_component(CMAKE_CUDA_HOST_COMPILER $ENV{CUDAHOSTCXX} PROGRAM)
   if(NOT EXISTS ${CMAKE_CUDA_HOST_COMPILER})