configure: Make NPTL non-optional

Now all linux-user targets support building with NPTL, we can make it
mandatory. This is a good idea because:
 * NPTL is no longer new and experimental; it is completely standard
 * in practice, linux-user without NPTL is nearly useless for
   binaries built against non-ancient glibc
 * it allows us to delete the rather untested code for handling
   the non-NPTL configuration

Note that this patch leaves the CONFIG_USE_NPTL ifdefs in the
bsd-user codebase alone. This makes no change for bsd-user, since
our configure test for NPTL had a "#include <linux/futex.h>"
which means bsd-user would never have been compiled with
CONFIG_USE_NPTL defined, and it still is not.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>