fix: remove not working method to change the caddy label-key

This was never actually used by caddy-docker-proxy so its better to just remove it.

plugins/caddy-vhosts/internal-functions

@@ -71,8 +71,3 @@ fn-caddy-tls-internal() {
   declare APP="$1"
   fn-plugin-property-get-default "caddy" "$APP" "tls-internal" "false"
 }
-
-fn-caddy-label-key() {
-  declare APP="$1"
-  fn-plugin-property-get-default "caddy" "$APP" "label-key" "caddy"
-}