Makefile: clean the right docs directory

The Makefile no longer uses `docs/Makefile` to build
the docs, so we shouldn't use it to clean them. This
patch updates the clean target to remove the right
directory.

Change-Id: I6746f338ddeb5c5a316b4df158d399574b5d0b8a
Tuleap: #749

Makefile

@@ -8,4 +8,4 @@ env:
 
 clean:
 	rm -rf env
-	make -C docs clean
+	rm -rf docs/build