package/glibc: remove the default value of the $(PKG)_SOURCE variable

Signed-off-by: Jerzy Grzegorek <jerzy.m.grzegorek@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/glibc/glibc.mk

@@ -7,7 +7,6 @@
 ifeq ($(BR2_arc),y)
 GLIBC_VERSION =  arc-2017.09-release
 GLIBC_SITE = $(call github,foss-for-synopsys-dwc-arc-processors,glibc,$(GLIBC_VERSION))
-GLIBC_SOURCE = glibc-$(GLIBC_VERSION).tar.gz
 else
 # Generate version string using:
 #   git describe --match 'glibc-*' --abbrev=40 origin/release/MAJOR.MINOR/master