sys: util.h: remove template implementation of ARRAY_SIZE

This was added because GNU G++ doesn't support the built-in function
used to produce a compile-time error when invalid parameters are
passed to the macro imlementation.  The template implementation does
not work on declarations like this:

  mytype array[] = { ... }

because it requires an explicit size for the template parameter.

Remove the template specialization, and support C++ with a version of
the macro that doesn't involve references to undefined builtins.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>