Remove MinGW from Appveyor

It stopped working due to missing Freetype for some reason. We already have a replacement on the cmake branch, so simply wait for that.

.appveyor.yml

@@ -1,8 +1,6 @@
 version: build.{build}
 
-# We run MinGW on the Visual Studio 2019 image and MSVC on the Visual Studio 2017 image.
 image:
-  - Visual Studio 2019
   - Visual Studio 2017
 
 environment:
@@ -13,10 +11,6 @@ environment:
       PLATFORM_NAME: win64
     - VS_GENERATOR: Visual Studio 15 2017
       PLATFORM_NAME: win32
-matrix:
-  exclude:
-    - image: Visual Studio 2019
-      PLATFORM_NAME: win32
 
 install:
 - cmd: |-
@@ -117,37 +111,6 @@ after_build:
 
     IF "%PLATFORM_NAME%"=="win64" 7z a RmlUi-%PLATFORM_NAME%-samples-only.zip .\Samples\* -r -xr!src\ -x!shell\ -x!luainvaders\
 
-for:
--
-  matrix:
-    only:
-      - image: Visual Studio 2019
-  install:
-  - cmd: |-
-      C:\msys64\usr\bin\bash -lc "pacman --needed --noconfirm -S mingw-w64-x86_64-gcc mingw-w64-x86_64-make mingw-w64-x86_64-freetype mingw-w64-x86_64-lua"
-      C:\msys64\usr\bin\bash -lc "pacman --needed --noconfirm -S mingw-w64-x86_64-sfml mingw-w64-x86_64-SDL2 mingw-w64-x86_64-SDL2_image mingw-w64-x86_64-glew"
-
-      mkdir Build
-      cd Build
-
-      set PATH=%PATH:C:\Program Files\Git\usr\bin;=%
-      set PATH=C:\msys64\mingw64\bin;C:\msys64\usr\bin;%PATH%
-      set GLEW_INCLUDE_DIR=C:/msys64/mingw64/include/GL
-      set GLEW_LIBRARIES=C:/msys64/mingw64/lib/libglew32.dll.a
-
-      cmake -G "MinGW Makefiles" -DBUILD_SHARED_LIBS=ON -DENABLE_PRECOMPILED_HEADERS=OFF -DBUILD_SAMPLES=ON -DBUILD_LUA_BINDINGS=ON -DWARNINGS_AS_ERRORS=OFF -DCMAKE_C_COMPILER=gcc.exe -DCMAKE_CXX_COMPILER=g++.exe -DCMAKE_MAKE_PROGRAM=mingw32-make.exe -DGLEW_INCLUDE_DIR=%GLEW_INCLUDE_DIR% -DGLEW_LIBRARIES=%GLEW_LIBRARIES% ..
-
-      set CHERE_INVOKING=yes
-      set MSYSTEM=MINGW64
-
-  build_script:
-  - cmd: |-
-      C:\msys64\usr\bin\bash -lc "mingw32-make.exe -j4"
-
-  after_build:
-  - cmd: |-
-      ls {*.exe,*.dll} -s -h -X
-
 artifacts:
 - path: RmlUi-*.zip
 deploy: