6rd: Remove sourcerouting parameter registration

Commit 31214c38 removes IPv6 unneeded source-dest-routing workarounds;
as a result sourcerouting parameter is unused and can be removed.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45939 3c298f89-4303-0410-b956-a3cf2f4a3e73