Remove workaround on VSTS that is no longer needed (#7666)

.vsts-ci/mac.yml

@@ -20,13 +20,6 @@ phases:
     displayName: Set Build Name for Non-PR
     condition: ne(variables['Build.Reason'], 'PullRequest')
 
-# Several scripts require TMPDIR to exist
-# Set it to AGENT_TEMPDIRECTORY so that we know it exists
-  - powershell: |
-      Write-Host "##vso[task.setvariable variable=TMPDIR]$env:AGENT_TEMPDIRECTORY"
-    displayName: Set TMPDIR to AGENT_TEMPDIRECTORY
-    condition: succeededOrFailed()
-
   - powershell: |
       git submodule update --init
     displayName: SubModule Init