Remove the ensure => absent uninstall code.

This is likely to be a controversial change so I wanted to put some
explanation of our reasoning into the commit message.  This gets
kind of complex so I'll start with the problem and then the reasoning.

Problem:

We rely heavily on the ability to uninstall and reinstall postgres
throughout our testing code, testing features like "can I move from the
distribution packages to the upstream packages through the module" and
over time we've learnt that the uninstall code simply doesn't work a lot
of the time.  It leaves traces of postgres behind or fails to remove
certain packages on Ubuntu, and generally causes bits to be left on your
system that you didn't expect.

When we then reinstall things fail because it's not a true clean slate,
and this causes us enormous problems during test.  We've spent weeks and
months working on these tests and they simply don't hold up well across
the full range of PE platforms.

Reasoning:

Due to all these problems we've decided to take a stance on uninstalling
in general.  We feel that in 2014 it's completely reasonable and normal
to have a good provisioning pipeline combined with your configuration
management and the "correct" way to uninstall a fully installed service
like postgresql is to simply reprovision the server without it in the
first place.  As a general rule this is how I personally like to work
and I think is a good practice.

WAIT A MINUTE:

We understand that there are environments and situations in which it's
not easy to do that.  What if you accidently deployed Postgres on
100,000 nodes?  When this work is finished I'm going to take a look at
building some example 'profiles' to be found under examples/ within this
module that can uninstall postgres on popular platforms.  These can be
modified and used in your specific case to uninstall postgresql.  They
will be much more brute force and reliant on deleting entire directories
and require you to do more work up front in specifying where things are
installed but we think it'll prove to be a much cleaner mechanism for
this kind of thing rather than trying to weave it into the main module
logic itself.