Remove unused code

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1934,7 +1934,6 @@ because xbuild doesn't support framework reference assemblies.
     <_ProguardConfiguration Include="$(ProguardConfigFiles)" />
   </ItemGroup>
   <ItemGroup Condition=" '$(ProguardConfigFiles)' == '' ">
-    <_ProguardConfiguration Include="$(_AndroidSdkDirectory)tools\proguard\proguard-android.txt" />
     <_ProguardConfiguration Include="$(IntermediateOutputPath)proguard\proguard_xamarin.cfg"         Condition=" '$(AndroidLinkTool)' != '' " />
     <_ProguardConfiguration Include="$(_ProguardProjectConfiguration)"                               Condition=" '$(AndroidLinkTool)' != '' " />
     <_ProguardConfiguration Include="$(IntermediateOutputPath)proguard\proguard_project_primary.cfg" Condition=" '$(AndroidLinkTool)' != '' " />