Fix dotnet feed publishing (#496)

Removing the steps that create AssetManifest files from Linux to prevent the Publish task from trying to upload the same assets multiple times in parallel and fail on file locks.
Removing empty parameter on macOS step.

azure-pipelines.yml

@@ -118,8 +118,6 @@ stages:
         - script: eng/common/cibuild.sh
             --configuration $(_BuildConfig)
             --prepareMachine
-            $(_OfficialBuildArgs)
-            /p:DotNetPublishUsingPipelines=$(_PublishUsingPipelines)
           displayName: Build
         - task: PublishBuildArtifacts@1
           displayName: Upload TestResults
@@ -151,7 +149,6 @@ stages:
         - script: eng/common/cibuild.sh
             --configuration $(_BuildConfig)
             --prepareMachine
-            $(_InternalRuntimeDownloadArgs)
           displayName: Build
         - task: PublishBuildArtifacts@1
           displayName: Upload TestResults