base: remove initcwnd if-up.d hook for >= oneiric

We currently have an if-up.d hook to set the TCP initial congestion
window to 10*MSS on all boxes, but this is currently unneeded on all of
our recent boxes. Linux upstream made 10 the default with commit
442b9635c, which is included since 2.6.39, way before any of our precise
systems.

As checking for a running kernel is kind of evil, make the check
conditional to lsbdistcodename, and remove the if-up hook in everything
post-oneiric (natty = 2.6.38, oneiric = 3.0).

Change-Id: Idfe83887b5601fe1c02766ea6d7121a2bf516ef4