configure: Don't say target_nptl="no" if there is no linux-user target

For architectures with no linux-user target, don't claim no NPTL
support. This has no behavioural change, but it means that we
won't accidentally add a new linux-user target without threading
support in future (because attempting to do so would be a compile
failure rather than a silent lack of support).

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>

configure

@@ -4206,7 +4206,6 @@ case "$target_name" in
   cris)
   ;;
   lm32)
-    target_nptl="no"
   ;;
   m68k)
     bflt="yes"
@@ -4235,7 +4234,6 @@ case "$target_name" in
     target_nptl="no"
   ;;
   moxie)
-    target_nptl="no"
   ;;
   or32)
     TARGET_ARCH=openrisc
@@ -4289,7 +4287,6 @@ case "$target_name" in
   ;;
   xtensa|xtensaeb)
     TARGET_ARCH=xtensa
-    target_nptl="no"
   ;;
   *)
     error_exit "Unsupported target CPU"