debian/control: Follow current policy for Priority field

Policy 2.5 explains:

 The priority of a package is determined solely by the functionality
 it provides directly to the user. The priority of a package should
 not be increased merely because another higher-priority package
 depends on it

and:

 extra
  This priority is deprecated. Use the optional priority instead.

This means we can treat liblzma5 and xzdec as optional now.
diff --git a/debian/changelog b/debian/changelog
index 36d20d9..58db300 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -15,6 +15,7 @@
       at run time.
       Closes: #919950.  Thanks to Josh Triplett.
     - Breaks: liblzma2 versions without symbol versioning.
+    - Priority: optional.
   * xz-utils:
     - Lower priority of xz-utils to standard.  Closes: #685203.
     - README.Debian: Remove notes about differences from upstream.
diff --git a/debian/control b/debian/control
index fc22bb0..6b4dbd2 100644
--- a/debian/control
+++ b/debian/control
@@ -1,6 +1,6 @@
 Source: xz-utils
 Section: utils
-Priority: required
+Priority: optional
 Maintainer: Jonathan Nieder <jrnieder@gmail.com>
 Uploaders: Mohammed Adnène Trojette <adn+deb@diwi.org>
 Build-Depends: debhelper (>= 8.9.0), dpkg-dev (>= 1.16.2),
@@ -59,7 +59,6 @@
   * support for flushing the encoder.
 
 Package: xzdec
-Priority: extra
 Architecture: any
 Depends: ${shlibs:Depends}, ${misc:Depends}
 Multi-Arch: foreign
@@ -88,7 +87,6 @@
 
 Package: liblzma-dev
 Section: libdevel
-Priority: optional
 Multi-Arch: same
 Architecture: any
 Depends: liblzma5 (= ${binary:Version}), ${misc:Depends}
@@ -105,7 +103,6 @@
 
 Package: liblzma-doc
 Section: doc
-Priority: optional
 Architecture: all
 Depends: ${misc:Depends}
 Suggests: liblzma-dev