libosip2: remove redundant LIBOSIP2_SOURCE definition

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libosip2/libosip2.mk

@@ -5,7 +5,6 @@
 #############################################################
 
 LIBOSIP2_VERSION = 3.6.0
-LIBOSIP2_SOURCE = libosip2-$(LIBOSIP2_VERSION).tar.gz
 LIBOSIP2_SITE = $(BR2_GNU_MIRROR)/osip
 LIBOSIP2_INSTALL_STAGING = YES