Do not try finding pip for python2 when not required
Do not try finding pip for python2 when not required

[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] 279a7589b03db69fd1b85d947cd0171dacef94ee
[2] http://lists.openstack.org/pipermail/openstack-discuss/2020-January/012182.html

Change-Id: I36ee53e57e468d760b80a7e621b90899867a8efd

openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack,nawawi/openstack