Fix missing removal of v-transition class

[posva]

Fix #2972

At first I added this test too since manually updating the directive may differ from real updates but then I realised it must be already tested on binding tests so I removed it.

  it('removes v-transition on dynamic transition', function (done) {
    var el = document.createElement('div')
    document.body.appendChild(el)
    var vm = new Vue({
      el: el,
      template: '<div :transition="trans"></div>',
      data: {
        trans: null
      },
      replace: true
    })

    _.nextTick(function () {
      expect(vm.$el.className).toBe('v-transition')
      vm.trans = 'a'
      _.nextTick(function () {
        expect(vm.$el.className).toBe('a-transition')
        done()
      })
    })
  })