refactor(bitswap) forwardN no longer needed

@whyrusleeping

now, the pubsub channel closes after sending N blocks. we got this
functionality for free from the fix. So, the forwardN wrap is no longer
required! woohoo

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>
misc(bs/n) rm dead code

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>