libstdc++: Fix up parallel_backend_serial.h [PR97549]

libstdc++: Fix up parallel_backend_serial.h [PR97549]

In GCC 10, parallel_backend.h just included parallel_backend_{serial,tbb}.h and
did nothing beyond that, and parallel_backend_tbb.h provided directly
namespace __pstl { namespace __par_backend { ... } }
and defined everything in there, while parallel_backend_serial.h did:
namespace __pstl { namespace __serial { ... } } and had this
namespace __pstl { namespace __par_backend { using namespace __pstl::__serial; } }
at the end.
In GCC 11, parallel_backend.h does:
namespace __pstl { namespace __par_backend = __serial_backend; }
after including parallel_backend_serial.h or
namespace __pstl { namespace __par_backend = __tbb_backend; }
after including parallel_backend_tbb.h.  The latter then has:
namespace __pstl { namespace __tbb_backend { ... } }
and no using etc. at the end, while parallel_backend_serial.h changed to:
namespace __pstl { namespace __serial_backend { ... } }
but has this leftover block from the GCC 10 times.  Even changing that
using namespace __pstl::__serial;
to
using namespace __pstl::__serial_backend;
doesn't work, as it clashes with
namespace __pstl { namespace __par_backend = __serial_backend; }
in parallel_backend.h.

2021-02-23  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/97549
	* include/pstl/parallel_backend_serial.h: Remove __pstl::__par_backend.