ipn/ipnserver: remove protoSwitchConn shenanigans; just use http.Server early

Now that everything's just HTTP, there's no longer a need to have a
header-sniffing net.Conn wraper that dispatches which route to
take. Refactor to just use an http.Server earlier instead.

Updates #6417

Change-Id: I12a2054db4e56f48660c46f81233db224fdc77cb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/proxyconnect_js.go

@@ -4,14 +4,8 @@
 
 package ipnserver
 
-import (
-	"bufio"
-	"context"
-	"net"
+import "net/http"
 
-	"tailscale.com/types/logger"
-)
-
-func (s *Server) handleProxyConnectConn(ctx context.Context, br *bufio.Reader, c net.Conn, logf logger.Logf) {
+func (s *Server) handleProxyConnectConn(w http.ResponseWriter, r *http.Request) {
 	panic("unreachable")
 }