Don't define NULL to nullptr in C++11 mode

While C++ does seem to permit this, both GCC and clang do not do this
in their bundled <stddef.h> headers. Instead, both use the null
builtin (and g++ unconditionally overrides NULL back to null).

Our local change makes FreeBSD's C++ environment subtly different from
the rest of the world such that code that compiles fine with clang++
on FreeBSD doesn't compile fine on other systems and doesn't compile
on FreeBSD using g++.

This reverts commit 3033610f5dfd03b68de35631a8693963a80099c2.

PR: 286477 (exp-run)
Reviewed by: olce, imp, dim, emaste
Differential Revision: https://reviews.freebsd.org/D50089