configure: remove useless write_c_skeleton

This is not needed ever since QEMU stopped detecting -liberty; this
happened with the Meson switch but it is quite likely that the
library was not really necessary years before.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -638,7 +638,6 @@ if test "$mingw32" = "yes" ; then
   EXESUF=".exe"
   # MinGW needs -mthreads for TLS and macro _MT.
   CONFIGURE_CFLAGS="-mthreads $CONFIGURE_CFLAGS"
-  write_c_skeleton;
   prefix="/qemu"
   bindir=""
   qemu_suffix=""