Remove Matcher field from webhook struct

[kristinapathak]

https://github.com/xmidt-org/webpa-common/blob/179b0f36052587995ad5927313fbec7b6e14be9c/webhook/webhook.go#L50

Moving forward, we only want to filter events based on the destination. Once this change is imported into scytale, tr1d1um, and caduceus, I believe we will need to increase their api version.