discontinue i21of4 (old Apple hardware);

--- a/src/Pure/Admin/isabelle_cronjob.scala	Fri Jun 25 21:33:21 2021 +0200
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Fri Jun 25 22:05:14 2021 +0200
@@ -209,6 +209,16 @@
 
   val remote_builds_old: List[Remote_Build] =
     List(
+      Remote_Build("Linux A", "i21of4", user = "i21isatest",
+        proxy_host = "lxbroy10", proxy_user = "i21isatest",
+        self_update = true,
+        options = "-m32 -M1x4,2,4" +
+          " -e ISABELLE_OCAML=ocaml -e ISABELLE_OCAMLC=ocamlc -e ISABELLE_OCAML_SETUP=true" +
+          " -e ISABELLE_GHC_SETUP=true" +
+          " -e ISABELLE_MLTON=mlton" +
+          " -e ISABELLE_SMLNJ=sml" +
+          " -e ISABELLE_SWIPL=swipl",
+        args = "-a -d '~~/src/Benchmarks'"),
       Remote_Build("Linux A", "lxbroy9",
         java_heap = "2g", options = "-m32 -B -M1x2,2", args = "-N -g timing"),
       Remote_Build("Linux Benchmarks", "lxbroy5", historic = true, history = 90,
@@ -303,16 +313,6 @@
   val remote_builds1: List[List[Remote_Build]] =
   {
     List(
-      List(Remote_Build("Linux A", "i21of4", user = "i21isatest",
-        proxy_host = "lxbroy10", proxy_user = "i21isatest",
-        self_update = true,
-        options = "-m32 -M1x4,2,4" +
-          " -e ISABELLE_OCAML=ocaml -e ISABELLE_OCAMLC=ocamlc -e ISABELLE_OCAML_SETUP=true" +
-          " -e ISABELLE_GHC_SETUP=true" +
-          " -e ISABELLE_MLTON=mlton" +
-          " -e ISABELLE_SMLNJ=sml" +
-          " -e ISABELLE_SWIPL=swipl",
-        args = "-a -d '~~/src/Benchmarks'")),
       List(Remote_Build("Linux B", "lxbroy10", historic = true, history = 90,
         options = "-m32 -B -M1x4,2,4,6", args = "-N -g timing")),
       List(Remote_Build("macOS 10.13 High Sierra", "lapbroy68",