Remove unneeded refs to System.Text.Json

Microsoft.Toolkit.Uwp.SampleApp/Microsoft.Toolkit.Uwp.SampleApp.csproj

@@ -124,9 +124,6 @@
     <PackageReference Include="Microsoft.Xaml.Behaviors.Uwp.Managed">
       <Version>2.0.1</Version>
     </PackageReference>
-    <PackageReference Include="System.Text.Json">
-      <Version>4.7.2</Version>
-    </PackageReference>
     <PackageReference Include="NotificationsVisualizerLibrary">
       <Version>1.0.5</Version>
     </PackageReference>

Microsoft.Toolkit.Uwp/Properties/Microsoft.Toolkit.Uwp.rd.xml

@@ -1,6 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<Directives xmlns="http://schemas.microsoft.com/netfx/2013/01/metadata">
-  <Library Name="Microsoft.Toolkit.Uwp">
-    <Namespace Name="System.Text.Json.Serialization.Converters" Dynamic="Required All"/>
-  </Library>
-</Directives>

UnitTests/UnitTests.UWP/UnitTests.UWP.csproj

@@ -122,9 +122,6 @@
     <PackageReference Include="System.Xml.XPath.XmlDocument">
       <Version>4.3.0</Version>
     </PackageReference>
-    <PackageReference Include="System.Text.Json">
-      <Version>4.7.2</Version>
-    </PackageReference>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Converters\Test_AdaptiveHeightValueConverter.cs" />