Move transitional package to to "oldlibs/optional"

This resolves two lintian warnings:

W: notmuch-emacs: transitional-package-should-be-oldlibs-optional oldlibs/extra
W: notmuch-emacs: priority-extra-is-replaced-by-priority-optional
---
 debian/control | 1 -
 1 file changed, 1 deletion(-)

diff --git a/debian/control b/debian/control
index 3a624fdc..bb88b0f7 100644
--- a/debian/control
+++ b/debian/control
@@ -113,7 +113,6 @@ Description: Ruby interface to the notmuch mail search and index library
 
 Package: notmuch-emacs
 Section: oldlibs
-Priority: extra
 Architecture: all
 Depends: elpa-notmuch, ${misc:Depends}
 Description: thread-based email index, search and tagging (transitional package)
-- 
2.15.0

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