remove unnecessary build targets in appveyor

Author: q4a

appveyor.yml

@@ -2,12 +2,9 @@ version: '{build}'
 pull_requests:
   do_not_increment_build_number: true
 skip_tags: true
-image:
-- Visual Studio 2017
-- Previous Visual Studio 2017
+image: Visual Studio 2017
 configuration:
 - Release
-- Mixed
 - Debug
 platform:
 - x86
@@ -20,4 +17,4 @@ build:
   verbosity: minimal
 after_build:
 - cmd: xrbuild.cmd
-test: off
+test: off