build: remove SourceFile and SourceURL from opkg metadata - they are useless...

build: remove SourceFile and SourceURL from opkg metadata - they are useless without the corresponding openwrt package directory

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39038 3c298f89-4303-0410-b956-a3cf2f4a3e73