cmd/cli: remove --webclient flag from up

Causing issues building a stable release. Getting rid of the flag
for now because it was only available in unstable, can still be
turned on through localapi.

A #cleanup

Co-authored-by: Will Norris <will@tailscale.com>
Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -905,7 +905,6 @@ func TestUpdatePrefs(t *testing.T) {
 				OperatorUserSet:           true,
 				RouteAllSet:               true,
 				RunSSHSet:                 true,
-				RunWebClientSet:           true,
 				ShieldsUpSet:              true,
 				WantRunningSet:            true,
 			},