[client] Drop an unreachable guard and fix two stale comments

- loginOverridesInput's nil-message guard cannot be reached: Login
  dereferences the message well before it, in storedLoginConfig.
- The docstring above afterLoginPreCheck described persistLoginOverrides,
  which lives further down the file and now carries its own.
- UpdateConfig's comment named DirectUpdateConfig; the function is
  DirectUpdateOrCreateConfig.

client/server/update_settings_gate.go

@@ -43,10 +43,6 @@ func configChangeRequested(stored *profilemanager.Config, input profilemanager.C
 // through this builder, so the gate can neither refuse a field the write
 // ignores nor miss one it applies.
 func loginOverridesInput(msg *proto.LoginRequest) profilemanager.ConfigInput {
-	if msg == nil {
-		return profilemanager.ConfigInput{}
-	}
-
 	preSharedKey := msg.OptionalPreSharedKey
 	if preSharedKey != nil && *preSharedKey == "" {
 		preSharedKey = nil