Drop duplicated BUILT_SOURCES in doc/Makefile.am
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 02831b1..401c1b3 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -59,8 +59,6 @@
 	confdir.sgml		\
 	func.refs		\
 	$(NULL)
-BUILT_SOURCES =			\
-	$(NULL)
 SUFFIXES =	\
 	.fncs	\
 	.sgml	\