Remove .appveyor.yml

I doubt we'll ever use appveyor again, so...

.appveyor.yml

@@ -1,23 +0,0 @@
-version: 1.0.{build}
-image: Visual Studio 2019
-
-platform: x64
-configuration: Debug
-
-build:
-  project: SpaceStation.sln
-  parallel: false
-  verbosity: minimal
-
-before_build:
-- cmd: py -3 -m pip install --user requests
-- cmd: py -3 RUN_THIS.py --no-prompt
-
-build_script:
-- ps: dotnet build SpaceStation14.sln /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" /p:AppVeyor=yes
-
-test:
-  assemblies:
-    only:
-    - bin/Content.IntegrationTests/Content.IntegrationTests.dll
-    - bin/Content.Tests/Content.Tests.dll