Do not delete tile cache on upgrade

On a production machine the tile cache might be several TB worth of data that can take weeks to re-create. Therefore the cache should only be deleted when tirex is actually removed from the machine, not when tirex is upgraded (which will also invoke the prerm script).

Caution: Since the prerm script invoked on upgrade is the prerm script of the OLD package, even after this fix has been deployed, upgrading to a new version from a pre-fix version WILL STILL DELETE YOUR CACHE unless you manually tweak /var/lib/dpkg/info/tirex.prerm before upgrading.