React to NuGet package pruning warnings (dotnet#12955)

Contributes to dotnet/sdk#46829

NuGet added a new feature that automatically prunes package and project references that are provided by the shared framework that is targeted.

Resolve the one warning that got emitted when building the repository in non-source-only mode.

Author: ViktorHofer

src/System.Private.Windows.Core/src/System.Private.Windows.Core.csproj

@@ -30,7 +30,6 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Windows.CsWin32" PrivateAssets="all" />
     <PackageReference Include="System.Formats.Nrbf" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" />
   </ItemGroup>
 
   <ItemGroup>

src/System.Private.Windows.GdiPlus/System.Private.Windows.GdiPlus.csproj

@@ -32,7 +32,6 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Windows.CsWin32" PrivateAssets="all" />
     <PackageReference Include="System.Formats.Nrbf" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" />
   </ItemGroup>
 
   <ItemGroup>

src/System.Windows.Forms.Primitives/src/System.Windows.Forms.Primitives.csproj

@@ -45,7 +45,6 @@
     <PackageReference Include="System.Configuration.ConfigurationManager" />
     <PackageReference Include="System.Resources.Extensions" />
     <PackageReference Include="System.Windows.Extensions" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" />
   </ItemGroup>
 
   <ItemGroup>

src/System.Windows.Forms/src/System.Windows.Forms.csproj

@@ -39,7 +39,6 @@
     <PackageReference Include="System.Configuration.ConfigurationManager" />
     <PackageReference Include="System.Resources.Extensions" />
     <PackageReference Include="System.Windows.Extensions" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" />
   </ItemGroup>
 
   <ItemGroup>