libstdc++: Remove std::__unicode::__null_sentinel

The name __null_sentinel is defined as a macro by newlib, so we can't
use it as an identifier. That variable is not actually used by
libstdc++, it was added because P2728R6 proposes std::uc::null_sentinel.
Since we don't need it and it breaks bootstrap for newlib targets, just
remove it. A null sentinel can still be used by constructing a
_Null_sentinel_t object as needed, rather than having a named object of
that type predefined.

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (__null_sentinel): Remove.
	* testsuite/17_intro/names.cc: Add __null_sentinel.