Remove unnecessary custom targets from AOAI (Azure#48350)

This custom `RequiredTargetFrameworks` appears to be unnecessarily overriding the common definition, preventing AOAI from building for .NET 8.

Author: joseharriaga

sdk/openai/Azure.AI.OpenAI/Directory.Build.props

@@ -20,8 +20,4 @@
     Add any shared properties you want for the projects under this package directory that need to be set before the auto imported Directory.Build.props
   -->
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory).., Directory.Build.props))\Directory.Build.props" />
-  
-  <PropertyGroup>
-    <RequiredTargetFrameworks Condition="$(IsTestProject) != 'true'">netstandard2.0</RequiredTargetFrameworks>
-  </PropertyGroup>
 </Project>