debian: Remove unneeded --sourcedir options from dh_install calls

With debhelper compatibility 7 and above, debian/tmp is used by default.

debian/changelog

@@ -9,6 +9,8 @@ dpkg (1.18.3) UNRELEASED; urgency=low
     - Get the reference build flags from dpkg-buildflags.pl, instead of
       hardcoding them, which might not match depending on the architecture.
       Closes: #794694
+  * Packaging:
+    - Remove unneeded --sourcedir options from dh_install calls.
   * Documentation:
     - Fix typos for --predep-package option name. Closes: #794688
 

debian/rules

@@ -102,7 +102,7 @@ endef
 binary-arch: install
 	dh_testdir -a
 	dh_testroot -a
-	dh_install --sourcedir=debian/tmp -a
+	dh_install -a
 	dh_installcron -a
 	dh_installlogrotate -a
 	dh_installchangelogs -a ChangeLog*
@@ -128,7 +128,7 @@ binary-arch: install
 binary-indep: install
 	dh_testdir -i
 	dh_testroot -i
-	dh_install --sourcedir=debian/tmp -i
+	dh_install -i
 	dh_installcron -i
 	dh_installchangelogs -i ChangeLog*
 	dh_installdocs -i