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
(cherry picked from commit 0a0f5f1bbe)

debian/changelog

@@ -3,8 +3,10 @@ notmuch (0.6~264) UNRELEASED; urgency=low
   * New snapshot, in progress
   * Fix description of notmuch-vim to mention vim, not emacs
     (Closes: #631974)
+  * Install zsh completion as an example instead of into /usr/share/zsh to
+    avoid file conflict with zsh.
 
- -- David Bremner <bremner@debian.org>  Thu, 23 Jun 2011 19:39:05 -0300
+ -- David Bremner <bremner@debian.org>  Thu, 30 Jun 2011 08:44:27 -0300
 
 notmuch (0.6~254) experimental; urgency=low
 

debian/notmuch.examples

@@ -0,0 +1 @@
+completion/notmuch-completion.zsh

debian/notmuch.install

@@ -1,4 +1,3 @@
 usr/bin
 usr/share/man/man1
 etc/bash_completion.d
-usr/share/zsh