Revert "install LIBS_FROM_GIT using python 2 and 3 where appropriate"

All the OpenStack projects should be able to run under Python 3 now so
the fallback installation of the Python 2 libraries should not be
needed any longer. This also avoids the problem of script files
installed by the libraries sometimes being overwritten by the Python 2
version leading to incorrect execution later, as discussed in
http://lists.openstack.org/pipermail/openstack-discuss/2019-September/009226.html

This reverts commit a2eb894.

Change-Id: I1cdb7e4a209872f1620be556b7278879a4b86df5

inc/python

@@ -380,16 +380,6 @@ function setup_dev_lib {
     fi
     local name=$1
     local dir=${GITDIR[$name]}
-    if python3_enabled; then
-        # Turn off Python 3 mode and install the package again,
-        # forcing a Python 2 installation. This ensures that all libs
-        # being used for development are installed under both versions
-        # of Python.
-        echo "Installing $name again without Python 3 enabled"
-        USE_PYTHON3=False
-        setup_develop $bindep $dir
-        USE_PYTHON3=True
-    fi
     setup_develop $bindep $dir
 }