control/controlclient: remove x/net/http2, use net/http

Saves 352 KB, removing one of our two HTTP/2 implementations linked
into the binary.

Fixes #17305
Updates #15015

Change-Id: I53a04b1f2687dca73c8541949465038b69aa6ade
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/deps_test.go

@@ -276,5 +276,8 @@ func TestMinTailscaledWithCLI(t *testing.T) {
 				}
 			}
 		},
+		BadDeps: map[string]string{
+			"golang.org/x/net/http2": "unexpected x/net/http2 dep; tailscale/tailscale#17305",
+		},
 	}.Check(t)
 }