Revert "Use virtualenv --seeder=pip so that libs are accessible"

--seeder=pip is breaking images where pip is not installed from source.

New virtualenv upstream release 20.0.2 has fixed the issue seen when
using sudo [1] by copying instead of symlinking and does not need this
fix anymore.

[1] https://github.com/pypa/virtualenv/commit/f4fd6a09916f26dc797a75698c53bbb95e48748e

This reverts commit be9530ae16.

Change-Id: I799982d9c023141cf612901084d4ecbd4447e969

nodepool/elements/infra-package-needs/install.d/40-install-bindep

@@ -22,5 +22,5 @@ if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
 fi
 set -e
 
-$DIB_PYTHON_VIRTUALENV --seeder=pip /usr/bindep-env
+$DIB_PYTHON_VIRTUALENV /usr/bindep-env
 /usr/bindep-env/bin/pip install bindep

nodepool/elements/nodepool-base/install.d/91-venv-os-testr

@@ -18,6 +18,5 @@ if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
 fi
 set -e
 
-# TODO is this used anymore?
-$DIB_PYTHON_VIRTUALENV --seeder=pip /usr/os-testr-env
+$DIB_PYTHON_VIRTUALENV /usr/os-testr-env
 /usr/os-testr-env/bin/pip install os-testr