FindIce: Remove unneeded search path modification

This change makes the search path ordering consistent; the code removed here was made redundant by the

```
      list(APPEND ice_roots "/opt/Ice-${ice_version}")
```

line earlier in the file, which adds these to the search order anyway.  Having the extra prepending which this PR removes can result in misconfiguration by misprioritising the search order.

At the request of upstream, I've also added support for the `find_package` `QUIET` option which suppresses messages if this is set.

Upstream PR (identical): http://public.kitware.com/pipermail/cmake-developers/2014-September/011224.html Staged: http://www.cmake.org/gitweb?p=cmake.git;a=shortlog;h=refs/heads/next 

--no-rebase

-------

Testing: all jobs will remain green.  The actual pathological ordering which this PR prevents is difficult to test unless you have a mixture of different Ice versions installed in /opt and other places, and the /opt Ice versions are of the same or similar version to the versions installed elsewhere and you have ICE_HOME pointing to a non-opt version.  Just run

```
cmake -DIce_DEBUG=ON /path/to/openmicroscopy"
```

and check the search ordering before and after merging this PR.