Removed AppVeyor CI.

.appveyor.yml

@@ -1,20 +0,0 @@
-shallow_clone: true
-
-os:
-  - Visual Studio 2019
-
-environment:
-  matrix:
-  - TOOLSET: vs2017
-  - TOOLSET: vs2019
-
-configuration:
-  - Debug
-  - Release
-
-install:
-  - git clone https://github.com/bkaradzic/bx ..\bx
-  - ..\bx\tools\bin\windows\genie --with-tools %TOOLSET%
-
-build:
-  project: .build/projects/$(TOOLSET)/bimg.sln