recv_block.h: remove seek_detached_blocks() prototype.

It's also in detached_block.h where it belongs.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
recv_block: don't segv on generation or json inject.

Introduced in 6851d627722418b3f2408971130c502bdee47c41.

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