Delete Dockerfile for unused Mariner testrunner image (#1343)

Author: mthalman

CODEOWNERS

@@ -23,7 +23,6 @@ src/azurelinux/**/opt/ @dotnet/runtime-infrastructure @dotnet/dotnet-docker-revi
 src/cbl-mariner/**/android/ @dotnet/runtime-infrastructure @dotnet/dotnet-docker-reviewers
 src/cbl-mariner/**/fpm/ @dotnet/runtime-infrastructure @dotnet/dotnet-docker-reviewers
 src/cbl-mariner/**/opt/ @dotnet/runtime-infrastructure @dotnet/dotnet-docker-reviewers
-src/cbl-mariner/**/docker-testrunner/ @dotnet/dotnet-docker-reviewers
 src/cbl-mariner/2.0/amd64/ @dotnet/runtime-infrastructure @dotnet/dotnet-docker-reviewers
 
 # centos

src/cbl-mariner/2.0/docker-testrunner/Dockerfile

@@ -3,31 +3,6 @@
     {
       "name": "dotnet-buildtools/prereqs",
       "images": [
-        {
-          "sharedTags": {
-            "cbl-mariner2.0-docker-testrunner": {}
-          },
-          "platforms": [
-            {
-              "dockerfile": "src/cbl-mariner/2.0/docker-testrunner",
-              "os": "linux",
-              "osVersion": "cbl-mariner2.0",
-              "tags": {
-                "cbl-mariner2.0-docker-testrunner-amd64": {}
-              }
-            },
-            {
-              "architecture": "arm64",
-              "dockerfile": "src/cbl-mariner/2.0/docker-testrunner",
-              "os": "linux",
-              "osVersion": "cbl-mariner2.0",
-              "tags": {
-                "cbl-mariner2.0-docker-testrunner-arm64v8": {}
-              },
-              "variant": "v8"
-            }
-          ]
-        },
         {
           "platforms": [
             {