debian: don't install zsh completion files into /usr/share/zsh

as of version 4.3.12, perhaps earlier, the Debian zsh package now
ships /usr/share/zsh/functions/Completion/Unix/_notmuch, so we
shouldn't install that ourselves anymore.

My understanding is that letting zsh ship the completion scripts is
the standard thing to do.

The script is still shipped in /usr/share/doc/notmuch/examples