meson: drop unnecessary -D legacy-x11=none

Array options are allowed to be empty, so there's no need to add an
extra value to convey that.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35309>

meson.options

@@ -780,10 +780,10 @@ option(
 option(
   'legacy-x11',
   type : 'array',
-  value : ['none'],
+  value : [],
   description : 'Build legacy X11 support features.',
   choices : [
-    'none', 'dri2'
+    'dri2'
   ],
 )