Drop configure probe for popen()

It was only used by the command encoder which we removed.

configure.ac

@@ -37,14 +37,6 @@ AC_DEFINE_UNQUOTED(SWIG_PLATFORM, ["$host"], [Platform that SWIG is built for])
 # For AC_EGREP_CPP
 AC_PROG_EGREP
 
-dnl Look for popen
-AC_ARG_WITH(popen, AS_HELP_STRING([--without-popen], [Disable popen]), with_popen="$withval")
-if test x"${with_popen}" = xno ; then
-AC_MSG_NOTICE([Disabling popen])
-else
-AC_CHECK_FUNC(popen, AC_DEFINE(HAVE_POPEN, 1, [Define if popen is available]), AC_MSG_NOTICE([Disabling popen]))
-fi
-
 dnl PCRE
 AC_ARG_WITH([pcre],
   [AS_HELP_STRING([--without-pcre],