Chorus effect inspector doesn't update voices automatically

[Kentarosan]

Godot version:
3.1

OS/device including version:
Windows 10

Issue description:
When changing the number of voices in the chorus audio effect in the inspector, the voices doesn't appears or disappears accordingly in the panel beneath.

Steps to reproduce:
Add the Chorus effect to an audio bus, click on it and modify the number of voices in the first parameter of the inspector.

[KoBeWi]

Duplicate of #20003