cmake: remove clang++ lookup in clang/generic.cmake

Fixes: #63771

The 'compiler/*/generic.cmake' is intended to set a C preprocessor which
can be used for devicetree preprocessing.

No C++ compiler is needed as host tool and should therefore not be set
in this file.

Remove the code to avoid setting a not required C++ compiler.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>