[dotnet] Don't import Microsoft.NET.Sdk's Sdk.props/Sdk.targets.

We're not in control of this anymore.

dotnet/targets/Xamarin.Shared.Sdk.props

@@ -1,6 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.props" />
   <Import Project="Xamarin.Shared.Sdk.Versions.props" />
 
   <PropertyGroup>

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -112,8 +112,6 @@
 		<DebuggerSupport Condition="'$(DebuggerSupport)' == ''">$(_BundlerDebug)</DebuggerSupport>
 	</PropertyGroup>
 
-	<Import Sdk="Microsoft.NET.Sdk" Project="Sdk.targets" />
-
 	<PropertyGroup>
 		<TargetPlatformSupported Condition=" '$(TargetPlatformIdentifier)' == '$(_PlatformName)' ">true</TargetPlatformSupported>
 	</PropertyGroup>