Once revert debhelper version change

Well, GitHub workflow specifies ubuntu 16.04 as test env. It is now under
EMS stage (https://lists.ubuntu.com/archives/ubuntu-announce/2021-March/000266.html)
but is still there. We can ignore that as "package build" env, but once
revert changes

debian/changelog

@@ -27,11 +27,8 @@ github-backup-utils (3.1.0~rc1) UNRELEASED; urgency=medium
     - Add Vcs-* metadata field.
     - Specify "Rules-Requires-Root: no"
     - Drop unnecessary "Build-Depends: devscripts"
-    - Bump debhelper 10 from 9 to reduce lintian warning.
     - Set Standards-Version: 4.5.1
     - Update Maintainer as Zachary Mark <zachary-mark@github.com>
-  * debian/compat
-    - Set 10 as above debhelper changes.
   * debian/{clean,manpages}
     - Add files to support manpages handles.
 

debian/compat

@@ -1 +1 @@
-10
+9

debian/control

@@ -3,7 +3,7 @@ Maintainer: Zachary Mark <zachary-mark@github.com>
 Section: misc
 Priority: optional
 Standards-Version: 4.5.1
-Build-Depends: debhelper (>= 10), git, moreutils, jq, rsync (>= 2.6.4), help2man,
+Build-Depends: debhelper (>= 9), git, moreutils, jq, rsync (>= 2.6.4), help2man,
 Homepage: https://github.com/github/backup-utils
 Vcs-Git: https://github.com/github/backup-utils.git
 Vcs-Browser: https://github.com/github/backup-utils