d/icedove.links: remove unneeded link to /u/s/i/e

We ship a useless link from
  /usr/lib/icedove/extension/{972ce4c6-7e08-4474-a285-3208198ce6fd}
to
  usr/share/icedove/extensions/{972ce4c6-7e08-4474-a285-3208198ce6fd}

This link isn't needed any longer and breaks packaging rules for Mozilla
related extensions.
Further informations on https://wiki.debian.org/Mozilla/ExtensionsPolicy

Closes: #638489