docs/Makefile.am: remove redundant variables

Remove DOC_SOURCE_DIR, introduced by and
unused since commit b325d741.

PERL is already detected in configure.ac.

docs/Makefile.am

@@ -16,11 +16,6 @@
 ## License along with this library.  If not, see
 ## <http://www.gnu.org/licenses/>.
 
-PERL = perl
-
-# The directory containing the source code (if it contains documentation).
-DOC_SOURCE_DIR=../src
-
 DEVHELP_DIR=$(datadir)/gtk-doc/html/libvirt
 
 apihtml =					\