Remove no-longer-needed Solaris 2.4 vfork bug workaround.

Remove no-longer-needed Solaris 2.4 vfork bug workaround.

The workaround was for improving performance on Solaris 2.4, but
is getting in the way now.  Emacs will still work if someone is
still running Solaris 2.4 in a museum somewhere; Sun dropped
support for Solaris 2.4 in 2003.
* configure.ac (ac_cv_func_vfork_works): Default to 'no' on
Solaris 2.4, so that AC_FUNC_VFORK doesn't think vfork works.
* src/callproc.c (Fcall_process) [HAVE_WORKING_VFORK]:
* src/process.c (create_process) [HAVE_WORKING_VFORK]:
Omit now-unnecessary workaround for the Solaris 2.4 vfork bug,
since Emacs no longer uses vfork on that platform.