Remove 7.3 Centos/RH from CI

Author: MattGal

buildpipeline/linux.groovy

@@ -47,9 +47,7 @@ simpleDockerNode('microsoft/dotnet-buildtools-prereqs:rhel7_prereqs_2') {
             // Get the user that should be associated with the submission
             def helixCreator = getUser()
             // Target queues
-            def targetHelixQueues = ['Centos.73.Amd64.Open',
-                                     'Centos.74.Amd64.Open',
-                                     'RedHat.73.Amd64.Open',
+            def targetHelixQueues = ['Centos.74.Amd64.Open',
                                      'RedHat.74.Amd64.Open',
                                      'Debian.87.Amd64.Open',
                                      'Ubuntu.1404.Amd64.Open',