resources: Remove the "arm64-ubuntu-18.04-img" resource

This was provided by ARM and does not conform to how we current build
disk images. Problems were noted in this commit:
https://gem5-review.googlesource.com/c/public/gem5/+/58910

Change-Id: If43220a7d82440c00c55cf6d5ae9c6f74e7a8665
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/59530
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/resources.json b/resources.json
index 2518336..b5d8bfc 100644
--- a/resources.json
+++ b/resources.json
@@ -121,19 +121,6 @@
         },
         {
             "type": "resource",
-            "name" : "arm64-ubuntu-18.04-img",
-            "documentation" : "A disk image containing Ubuntu 18.04 for arm64 which will run an `m5 readfile` instruction after booting. If no script file is specified an `m5 exit` instruction will be exected.",
-            "architecture" : "ARM",
-            "is_zipped" : true,
-            "md5sum" : "ec7d0156366743fa7b93cbfcea82925c",
-            "url" : "{url_base}/images/arm/ubuntu-18-04/arm64-ubuntu-20220425.img.gz",
-            "source" : null,
-            "additional_metadata" : {
-                "root_partition": "1"
-            }
-        },
-        {
-            "type": "resource",
             "name" : "arm64-ubuntu-20.04-img",
             "documentation" : "A disk image containing Ubuntu 18.04 for arm64 which will run an `m5 readfile` instruction after booting. If no script file is specified an `m5 exit` instruction will be exected.",
             "architecture" : "ARM",