Remove TestTFMs for netcore50 as well for those projects

Commit migrated from https://github.com/dotnet/corefx/commit/14a16de4737a59e1bedbf3cfdfe7781c29e83d2a

src/libraries/System.Private.Xml.Linq/tests/System.Private.Xml.Linq.Tests.builds

@@ -3,7 +3,7 @@
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
   <ItemGroup>
     <Project Include="axes\System.Xml.Linq.Axes.Tests.csproj" >
-      <TestTFMs>netcore50;net46</TestTFMs>
+      <TestTFMs>net46</TestTFMs>
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="events\System.Xml.Linq.Events.Tests.csproj" >

src/libraries/System.Private.Xml/tests/System.Private.Xml.Tests.builds

@@ -15,7 +15,7 @@
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="Readers\NameTable\System.Xml.RW.NameTable.Tests.csproj" >
-      <TestTFMs>netcore50;net46</TestTFMs>
+      <TestTFMs>net46</TestTFMs>
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="Readers\ReaderSettings\System.Xml.RW.ReaderSettings.Tests.csproj" >
@@ -63,11 +63,11 @@
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="XmlSchema\XmlSchemaSet\System.Xml.XmlSchemaSet.Tests.csproj" >
-      <TestTFMs>netcore50;net46</TestTFMs>
+      <TestTFMs>net46</TestTFMs>
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="XmlSchema\XmlSchemaValidatorApi\System.Xml.XmlSchema.XmlSchemaValidatorApi.Tests.csproj" >
-      <TestTFMs>netcore50;net46</TestTFMs>
+      <TestTFMs>net46</TestTFMs>
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="Xslt\XslTransformApi\System.Xml.Xsl.XslTransformApi.Tests.csproj" >
@@ -79,7 +79,7 @@
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="XPath\XPathDocument\System.Xml.XPath.Tests.csproj" >
-      <TestTFMs>netcore50;net46</TestTFMs>
+      <TestTFMs>net46</TestTFMs>
       <OSGroup>Windows_NT</OSGroup>
     </Project>
     <Project Include="XPath\XmlDocument\System.Xml.XPath.XmlDocument.Tests.csproj" >