tdb: remove lock ops

Now the transaction code uses the standard allrecord lock, that stops
us from trying to grab any per-record locks anyway.  We don't need to
have special noop lock ops for transactions.

This is a nice simplification: if you see brlock, you know it's really
going to grab a lock.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>