doc/go1.14: remove TODO about Dragonfly passing
    
    Both the Dragonfly release and tip builder have been passing for a
    while. The net package's interface API is working on both builders since
    CL 202317 which has been re-vendored in CL 202438.
    
    Updates #34368
    Updates #36878
    
    Change-Id: I187178b3a59f2604187af453207fb4e24a56105c
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217358
    Reviewed-by: Ian Lance Taylor <iant@golang.org>