debian: numpy is no longer an indirect dependency

With the autobahn/crossbar dependencies removed, numpy is no longer an
indirect dependency of labgrid.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

debian/rules

@@ -13,4 +13,4 @@ override_dh_virtualenv:
 		--upgrade-pip \
 		--extras deb \
 		--extra-pip-arg='--no-binary' \
-		--extra-pip-arg='cffi,numpy'
+		--extra-pip-arg='cffi'