Remove pip upgrade from windows sdist job (#879)

The windows sdist install jobs have started failing on a permission
issue when trying to upgrade pip prior to installing from sdist. This
commit removes the pip upgrade because the base windows azure pipelines
image is now including pip 20.2.1 which is sufficiently new (being >19)
and upgrading isn't explicitly needed. This should avoid the potential
permissions issue replacing the pip.exe file on disk already.

azure-pipelines.yml

@@ -174,7 +174,7 @@ stages:
               set -x
               set -e
               source activate qiskit-aer
-              pip install -U pip setuptools virtualenv wheel
+              pip install -U setuptools virtualenv wheel
               pip install dist/*tar.gz
               pip install git+https://github.com/Qiskit/qiskit-terra
               python tools/verify_wheels.py