Remove redundant site-packages cleanup steps (heroku#1007)

Since `get-pip.py` / pip will automatically detect and remove old
pip/setuptools versions if needed, so removing them manually is both not
necessary and slows down the build in the case where the pip version
changed, but setuptools remained the same.

Author: edmorley

bin/steps/python

@@ -167,10 +167,6 @@ if [ "$FRESH_PYTHON" ] || [[ ! $(pip --version) == *${PIP_VERSION}* ]]; then
 
   puts-step "Installing pip ${PIP_VERSION}, setuptools ${SETUPTOOLS_VERSION} and wheel ${WHEEL_VERSION}"
 
-  # Remove old installations.
-  rm -fr /app/.heroku/python/lib/python*/site-packages/pip-*
-  rm -fr /app/.heroku/python/lib/python*/site-packages/setuptools-*
-
   /app/.heroku/python/bin/python "$GETPIP_PY" pip=="${PIP_VERSION}" "setuptools==${SETUPTOOLS_VERSION}" "wheel==${WHEEL_VERSION}" &> /dev/null
 fi