[GTK#] Burn in fire, fire, fire, you're no longer required

appveyor.yml

@@ -16,9 +16,7 @@ environment:
 init:
 - ps: if (Test-Path env:nuget_address) {[System.IO.File]::AppendAllText("C:\Windows\System32\drivers\etc\hosts", "`n$($env:nuget_address)`tapi.nuget.org")}
 install:
-  - if not exist gtk-sharp-2.12.26.msi appveyor DownloadFile http://download.xamarin.com/GTKforWindows/Windows/gtk-sharp-2.12.26.msi
   - if not exist dotnet-2.0.0.exe appveyor DownloadFile https://download.microsoft.com/download/0/F/D/0FD852A4-7EA1-4E2A-983A-0484AC19B92C/dotnet-sdk-2.0.0-win-x64.exe -FileName "dotnet-2.0.0.exe"
-  - ps: Start-Process -FilePath "msiexec" -ArgumentList "/i gtk-sharp-2.12.26.msi /quiet /qn /norestart" -Wait
   - ps: Start-Process -FilePath "dotnet-2.0.0.exe" -ArgumentList "/quiet" -Wait
   - cmd: set PATH=%programfiles(x86)%\GtkSharp\2.12\bin\;%PATH%
 before_build:
@@ -36,5 +34,4 @@ artifacts:
   - path: artifacts\zip\*.zip
   - path: artifacts\inspectcode.xml
 cache:
-  - gtk-sharp-2.12.26.msi
   - dotnet-2.0.0.exe

docs/guidelines/build.md

@@ -4,12 +4,6 @@
 
 Avalonia requires at least Visual Studio 2017 and .NET Core SDK 2.0 to build on Windows.
 
-### Install GTK Sharp
-
-For the moment under windows, you must have [gtk-sharp](http://www.mono-project.com/download/#download-win)
-installed. Note that after installing the package your machine may require a restart before GTK# is
-added to your path. We hope to remove or make this dependency optional at some point in the future.
-
 ### Clone the Avalonia repository
 
 ```