Remove .NET Core 3.0 from test battery

Author: AArnott

azure-pipelines.yml

@@ -18,8 +18,6 @@ resources:
     image: mcr.microsoft.com/dotnet/core/sdk:3.1-bionic
   - container: focal
     image: mcr.microsoft.com/dotnet/core/sdk:3.1-focal
-  - container: disco-3.0
-    image: mcr.microsoft.com/dotnet/core/sdk:3.0-disco
   - container: archlinux
     image: andrewarnott/archlinux
 
@@ -274,8 +272,6 @@ stages:
           containerImage: bionic
         Ubuntu_Focal:
           containerImage: focal
-        Ubuntu_Disco_3_0:
-          containerImage: disco-3.0
         # Arch_Linux:
         #   containerImage: archlinux
         #   configureContainerCommand: 'sudo pacman -Sy --noconfirm git dotnet-sdk openssl-1.0'