Drop the kcp-go project

It isn't a real project, it has only one patch which seems not to be used anywhere.

According to the commit that added it (8547db1c)

commit 8547db1c629adf1bbdabcb44e2245944920472e6
Author: David Fifield <censored!>
Date:   Mon Feb 10 16:06:08 2020 -0700

    Bug 34043: Update snowflake to persist sessions across proxies
    
    This brings in changes related to
     #33745 Merge a turbotunnel branch
     #33897 Remove buffering from WebRTCPeer
     #34042 Reduce DataChannelTimeout
    
    The new dependencies kcp-go and smux form the new inner session layer.
    There's a patch to kcp-go to remove dependencies required for features
    we don't use.

However, the project files have been removed while doing #40849 (closed).

Probably, since we're using go.sum, removing dependencies is less of a problem now.