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.


--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.