gossip_ldebug is unused.

gossip_lerr is used in two places.  The messages are unique so line
numbers are unnecessary.

Also remove support for compiling gossip messages out.  It wasn't
possible to enable it anyway.
Signed-off-by: NMartin Brandenburg <martin@omnibond.com>
Signed-off-by: NMike Marshall <hubcap@omnibond.com>