debian: fix desktop install

Previous version expected full upstream install to be run, and also
caused lintian whine about the the desktop file being in a different
package than the script. I'm not sure they shouldn't both be in
elpa-notmuch, but I can see how they should be together.

debian/changelog

@@ -1,4 +1,4 @@
-notmuch (0.29-1) experimental; urgency=medium
+notmuch (0.29-2) experimental; urgency=medium
 
   * New upstream feature release. See /usr/share/doc/notmuch/NEWS.gz for
     details.

debian/elpa-notmuch.install

@@ -1 +0,0 @@
-usr/share/applications/notmuch-emacs-mua.desktop

debian/notmuch.install

@@ -2,3 +2,4 @@ usr/bin/notmuch
 usr/bin/notmuch-emacs-mua
 usr/share/bash-completion
 usr/share/zsh/vendor-completions
+emacs/notmuch-emacs-mua.desktop usr/share/applications