[xabuild] remove SRM reference

This appears to fix the build on macOS, we had this workaround from a mono bump in the past. Since xabuild has its own version of System.Reflection.Metadata that was already loaded, we weren't loading the one we are using in XA's MSBuild tasks.

Things appear to work without the reference now.

tools/xabuild/xabuild.csproj

@@ -37,9 +37,6 @@
     <Reference Include="MSBuild">
       <HintPath>$(MSBuildReferencePath)\MSBuild.$(_MSBuildExtension)</HintPath>
     </Reference>
-    <Reference Include="System.Reflection.Metadata" Condition=" '$(OS)' != 'Windows_NT' ">
-      <HintPath>$(MSBuildReferencePath)\System.Reflection.Metadata.dll</HintPath>
-    </Reference>
     <Reference Include="System" />
     <Reference Include="System.Core" />
     <Reference Include="System.Xml" />