[1] stopped installing pip for py2 when py3 is being used.
This patch makes sure we check only for py3 pip then.
Also removed some no-longer-relevant comment and
made uninstall behave the same.
Check for pip>=6 removed too.
See also [2].
[1] 279a7589b0
[2] http://lists.openstack.org/pipermail/openstack-discuss/2020-January/012182.html
Change-Id: I36ee53e57e468d760b80a7e621b90899867a8efd