revert: fix(transitions): on.leave can be undefined
revert: fix(transitions): on.leave can be undefined

This reverts commit 309859033

see ad1f6ce8c

vuetifyjs/vuetify,vuetifyjs/vuetify,Webifi/vuetify,Webifi/vuetify,Webifi/vuetify,vuetifyjs/vuetify,vuetifyjs/vuetify