[appveyor] Drop Cygwin builds again

They are so slow and we had only a couple of Cygwin build failure
reported in ~5 years.

appveyor.yml

@@ -28,14 +28,6 @@ environment:
       MINGW_CHOST: i686-w64-mingw32
       MSYS2_ARCH: i686
 
-    - compiler: cygwin
-      CYGWIN_PREFIX: C:\Cygwin64
-      CYGWIN_ARCH: x86_64
-   # Lots of test failures here!
-   #- compiler: cygwin
-   #  CYGWIN_PREFIX: C:\Cygwin
-   #  CYGWIN_ARCH: x86
-
 
 install:
 # - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "pacman --noconfirm --force -Sy && pacman --noconfirm --force -S pacman-mirrors && pacman --force -Syu --noconfirm"'
@@ -59,9 +51,6 @@ build_script:
   - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "curl https://raw.githubusercontent.com/mirror/mingw-w64/023eb04c396d4e8d8fcf604cfababc53dae13398/mingw-w64-headers/include/dwrite_1.h > %MINGW_PREFIX%/%MINGW_CHOST%/include/dwrite_1.h"'
   - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER; PATH=$PATH:/mingw64/bin:/mingw32/bin; ./autogen.sh --with-uniscribe --with-freetype --with-glib --with-gobject --with-cairo --with-icu --with-graphite2 --with-directwrite --build=%MINGW_CHOST% --host=%MINGW_CHOST% --prefix=%MINGW_PREFIX%; make -j3 check || .ci/fail.sh"'
 
-  - 'if "%compiler%"=="cygwin" curl https://raw.githubusercontent.com/mirror/mingw-w64/023eb04c396d4e8d8fcf604cfababc53dae13398/mingw-w64-headers/include/dwrite_1.h -o %CYGWIN_PREFIX%\usr\include\dwrite_1.h'
-  - 'if "%compiler%"=="cygwin" %CYGWIN_PREFIX%\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER; ./autogen.sh --with-uniscribe --with-freetype --with-glib --with-gobject --with-cairo --with-icu --with-graphite2 --with-directwrite; make -j3 check || .ci/fail.sh"'
-
 cache:
   - c:\tools\vcpkg\installed\
   - '%CYGWIN_PREFIX%\var\cache\setup'