package/network-manager: drop unrecognized --without-docs

--without-docs was not recognized since at least version 1.8.4, see:
http://autobuild.buildroot.org/results/e1464adf0069952ef0561a0a9b501d26bd409c4a

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/network-manager/network-manager.mk

@@ -26,7 +26,6 @@ NETWORK_MANAGER_CONF_OPTS = \
 	--disable-tests \
 	--disable-qt \
 	--disable-more-warnings \
-	--without-docs \
 	--with-crypto=gnutls \
 	--with-iptables=/usr/sbin/iptables \
 	--disable-ifupdown