.Net: Remove AzureOpenAIFileService (#7195)

### Motivation and Context
It was decided not to migrate existing `OpenAIFileService` to the
Azure.AI.OpenAI SDK v2 and eventually deprecate it.

### Description
This PR removes files related to the `OpenAIFileService` added by a
previous PR.

dotnet/src/Connectors/Connectors.AzureOpenAI/Connectors.AzureOpenAI.csproj

@@ -21,26 +21,10 @@
     <Description>Semantic Kernel connectors for Azure OpenAI. Contains clients for text generation, chat completion, embedding and DALL-E text to image.</Description>
   </PropertyGroup>
 
-  <ItemGroup>
-    <Compile Remove="Core\ClientCore.File.cs" />
-    <Compile Remove="Model\AzureOpenAIFilePurpose.cs" />
-    <Compile Remove="Model\AzureOpenAIFileReference.cs" />
-    <Compile Remove="Services\AzureOpenAIFileService.cs" />
-    <Compile Remove="Settings\AzureOpenAIFileUploadExecutionSettings.cs" />
-  </ItemGroup>
-
   <ItemGroup>
     <InternalsVisibleTo Include="SemanticKernel.Connectors.AzureOpenAI.UnitTests" />
   </ItemGroup>
 
-  <ItemGroup>
-    <None Include="Core\ClientCore.File.cs" />
-    <None Include="Model\AzureOpenAIFilePurpose.cs" />
-    <None Include="Model\AzureOpenAIFileReference.cs" />
-    <None Include="Services\AzureOpenAIFileService.cs" />
-    <None Include="Settings\AzureOpenAIFileUploadExecutionSettings.cs" />
-  </ItemGroup>
-
   <ItemGroup>
     <PackageReference Include="Azure.AI.OpenAI" VersionOverride="2.0.0-beta.2" />
   </ItemGroup>