net/wsconn: remove homegrown wrapper for turning a websocket.Conn into a net.Conn

The one from the nhooyr/websocket package seems to work equally well.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tailscale/depaware.txt

@@ -31,7 +31,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         go4.org/unsafe/assume-no-moving-gc                           from go4.org/intern
    W 💣 golang.zx2c4.com/wireguard/windows/tunnel/winipcfg           from tailscale.com/net/interfaces+
         inet.af/netaddr                                              from tailscale.com/cmd/tailscale/cli+
-   L    nhooyr.io/websocket                                          from tailscale.com/derp/derphttp+
+   L    nhooyr.io/websocket                                          from tailscale.com/derp/derphttp
    L    nhooyr.io/websocket/internal/errd                            from nhooyr.io/websocket
    L    nhooyr.io/websocket/internal/xsync                           from nhooyr.io/websocket
         tailscale.com                                                from tailscale.com/version
@@ -62,7 +62,6 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/net/tlsdial                                    from tailscale.com/derp/derphttp
         tailscale.com/net/tsaddr                                     from tailscale.com/net/interfaces+
      💣 tailscale.com/net/tshttpproxy                                from tailscale.com/derp/derphttp+
-   L    tailscale.com/net/wsconn                                     from tailscale.com/derp/derphttp
         tailscale.com/paths                                          from tailscale.com/cmd/tailscale/cli+
         tailscale.com/safesocket                                     from tailscale.com/cmd/tailscale/cli+
         tailscale.com/syncs                                          from tailscale.com/net/interfaces+