Remove dep on rdfpipe (from OpenSUSE)

--- configure.ac
+++ configure.ac
@@ -37,10 +37,4 @@ if test "x$HAVE_RAPPER" = "xno"; then
   AC_MSG_ERROR(You need the tool `rapper' from the `raptor-utils' package in order to compile Zeitgeist)
 fi
 
-# check for rdflib
-AC_CHECK_PROG(HAVE_RDFLIB, rdfpipe, yes, no)
-if test "x$HAVE_RDFLIB" = "xno"; then
-  AC_MSG_ERROR(You need the tool `rdfpipe' from the `python-rdflib' package in order to compile Zeitgeist)
-fi
-
 AC_OUTPUT