CI: Remove VS2015 AppVeyor build

.appveyor.yml

@@ -8,8 +8,6 @@ clone_depth: 50
 image:
   # Windows Server 2019
   - Visual Studio 2019
-  # Windows Server 2012 R2
-  - Visual Studio 2015
 
 environment:
   compiler: mingw
@@ -37,12 +35,6 @@ matrix:
       compiler_path: mingw32
       mingw_prefix: mingw-w64-i686
       hardening: --enable-all-bugs-are-fatal
-    - image: Visual Studio 2015
-      target: x86_64-w64-mingw32
-      compiler_path: mingw64
-      mingw_prefix: mingw-w64-x86_64
-      # hardening doesn't work with mingw-w64-x86_64-gcc, because it's gcc 8
-      hardening: --disable-gcc-hardening
 
 install:
 - ps: >-