Drop debian/compat and switch to debhelper-compat approach

Fixes:

| dh_clean
| dh_clean: error: Compatibility levels before 7 are no longer supported (level 5 requested)

Also replace `dh_clean -k` with `dh_prep`, to then fix:

| dh_clean -k
| dh_clean: warning: The -k option is not supported in compat 12; use dh_prep instead
| dh_clean: error: This feature was removed in compat 12.

debian/control

@@ -2,7 +2,7 @@ Source: grml-tips
 Section: grml
 Priority: optional
 Maintainer: Michael Prokop <mika@grml.org>
-Build-Depends: debhelper (>= 5)
+Build-Depends: debhelper-compat (= 12),
 Standards-Version: 3.9.5
 Homepage: http://git.grml.org/?p=grml-tips.git
 Vcs-git: git://git.grml.org/grml-tips.git

debian/rules

@@ -21,7 +21,7 @@ clean:
 install: build
 	dh_testdir
 	dh_testroot
-	dh_clean -k 
+	dh_prep
 	dh_installdirs usr/bin usr/share/grml-tips usr/lib/grml-tips
 
 	install -m 755 grml-tips debian/grml-tips/usr/bin/grml-tips