remove VS2013 build

appveyor.yml

@@ -14,7 +14,6 @@ matrix:
   fast_finish: true
     
 image:
-  - Visual Studio 2013
   - Visual Studio 2015
   - Visual Studio 2017
   - Visual Studio 2019
@@ -30,7 +29,6 @@ install:
   - set PATH=C:\Ruby24-x64\bin;%PATH%
   - set CMAKE_DEFINES -DASSIMP_WERROR=ON
   - if [%COMPILER%]==[MinGW] set PATH=C:\MinGW\bin;%PATH%
-  - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2013" set CMAKE_GENERATOR_NAME=Visual Studio 12 2013
   - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2015" set CMAKE_GENERATOR_NAME=Visual Studio 14 2015
   - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2017" set CMAKE_GENERATOR_NAME=Visual Studio 15 2017
   - if "%APPVEYOR_BUILD_WORKER_IMAGE%"=="Visual Studio 2019" set CMAKE_GENERATOR_NAME=Visual Studio 16 2019