Removing ImageSharp from .NET bindings

There is no longer a need for ImageSharp, since the .NET bindings will no
longer attempt to provide the ability to save screenshots to different
image file formats in the .NET Core case. This commit removes the
reference to the assembly and the assembly from the third_party dependency
directory.

Author: jimevans

dotnet/src/webdriver/WebDriver.NetCore.csproj

@@ -73,9 +73,6 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
-    <Reference Include="ImageSharp">
-      <HintPath>..\..\..\third_party\dotnet\imagesharp-1.0.0-alpha9-00194\netstandard1.3\ImageSharp.dll</HintPath>
-    </Reference>
     <Reference Include="Newtonsoft.Json">
       <HintPath>..\..\..\third_party\dotnet\json-net-10.0r2\netstandard1.3\Newtonsoft.Json.dll</HintPath>
     </Reference>