samples: cpp: Relocate samples/subsys/cpp to samples/cpp

This commit moves the C++ samples under `samples/subsys/cpp` to
`samples/cpp` because the C++ language and standard library support is
not a "subsystem" (aka. API) in conventional sense and the samples
demonstrating them should belong directly under the top samples
directory as does POSIX support.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>