Remove unused THREADS_NONE

Thread implementation is mandatory and configure will abort. So there is
no point in having a THREADS_NONE conditional/define.

configure.ac

@@ -2069,13 +2069,11 @@ AS_IF([test x$have_threads = xposix], [
            AC_DEFINE([THREADS_WIN32], [1], [Use w32 threads])
 	   g_threads_impl="WIN32"
 ], [
-           AC_DEFINE([THREADS_NONE], [1], [Use no threads])
 	   g_threads_impl="NONE"
            G_THREAD_LIBS=error
 ])
 AM_CONDITIONAL(THREADS_POSIX, [test "$g_threads_impl" = "POSIX"])
 AM_CONDITIONAL(THREADS_WIN32, [test "$g_threads_impl" = "WIN32"])
-AM_CONDITIONAL(THREADS_NONE, [test "$g_threads_impl" = "NONE"])
 
 if test "x$G_THREAD_LIBS" = xerror; then
         AC_MSG_ERROR($LIBS_NOT_FOUND_1$have_threads$LIBS_NOT_FOUND_2)

meson.build

@@ -1772,7 +1772,6 @@ have_sh = find_program('sh', required : false).found() # For glib-gettextize
 # (we add them for now to minimise the diff)
 glib_conf.set('HAVE_DLFCN_H', 1)
 glib_conf.set('STDC_HEADERS', 1)
-# THREADS_NONE
 glib_conf.set('SIZEOF___INT64', 8)
 
 # FIXME: How to detect Solaris? https://github.com/mesonbuild/meson/issues/1578