appveyor: Remove Visual Studio 2015 image tests

This image uses Windows Server 2012 which Libuv no longer supports (as of Libuv 1.50.0), since Libuv 1.50.0 depends on `SetThreadDescription`/`GetThreadDescription`

Author: squeek502

appveyor.yml

@@ -1,5 +1,4 @@
 image:
-  - Visual Studio 2015
   - Visual Studio 2022
 
 platform:
@@ -23,9 +22,6 @@ matrix:
     # Skip x86 for LuaRocks tests
     - platform: x86
       LUAROCKS_VER: 3.8.0
-    # Only test LuaRocks with latest MSVC
-    - image: Visual Studio 2015
-      LUAROCKS_VER: 3.8.0
 
 cache:
   - c:\lua -> appveyor.yml