simplify three false conditions for r_vector string

Is there a better condition we could use, e.g. assert something true rather than three things false? https://github.com/r-lib/cpp11/blob/main/inst/include/cpp11/r_vector.hpp#L1395

Still using 3 conditions, I think it is simpler to ensure that C is not constructible from SEXP, and neither C nor T is a std::string.
