Azure Pipelines: Drop useVenv

This wasn't actually ever used, since the value was always '$(useVenv)'.

.azure-pipelines/jobs/test-windows.yml

@@ -15,14 +15,12 @@ jobs:
       Python27-x64:
         python.version: '2.7'
         python.architecture: x64
-        useVenv: true
       Python35-x64:
         python.version: '3.5'
         python.architecture: x64
       Python36-x64:
         python.version: '3.6'
         python.architecture: x64
-        useVenv: true
       Python37-x64:
         python.version: '3.7'
         python.architecture: x64
@@ -35,7 +33,6 @@ jobs:
   - template: ../steps/run-tests-windows.yml
     parameters:
       runIntegrationTests: true
-      useVenv: '$(useVenv)'
 
 - job: Test_Secondary
   displayName: Test Secondary

.azure-pipelines/steps/run-tests-windows.yml

@@ -1,6 +1,5 @@
 parameters:
   runIntegrationTests:
-  useVenv: false
 
 steps:
 - task: UsePythonVersion@0
@@ -44,11 +43,8 @@ steps:
       # https://bugs.python.org/issue18199
       $env:TEMP = "R:\Temp"
 
-      tox -e py -- $env:USE_VENV_ARG -m integration -n auto --duration=5 --junit-xml=junit/integration-test.xml
+      tox -e py -- -m integration -n auto --duration=5 --junit-xml=junit/integration-test.xml
     displayName: Tox run integration tests
-    env:
-      ${{ if eq(parameters.useVenv, 'true') }}:
-        USE_VENV_ARG: "--use-venv"
 
 - task: PublishTestResults@2
   displayName: Publish Test Results