fix(VMenu): disabled activatorFixed when attach is enabled

[HIROYUKI20200507]

Description

• [Bug Report][2.6.4] v-menu + dynamic activator, activatorFixed prop does not reset  #14922

Markup:

▼packages/vuetify/dev/Playground.vue

<template>
  <div id="app">
    <v-app id="inspire">
      <div
        :style="{
          display: 'flex',
          flexDirection: 'column',
          alignItems: 'center'
        }"
      >
        <v-btn @click="attachInPlace = !attachInPlace">
          Toggle Attach
        </v-btn>
        <div :style="{ position: 'fixed', top: '50px' }">
          <v-menu :attach="attachInPlace ? $refs.container : false" offset-y>
            <template #activator="{ on }">
              <v-btn v-on="on">
                Menu
              </v-btn>
            </template>
            <v-list>
              <v-list-item v-for="item in items" :key="item">
                <v-list-item-title>
                  {{ item }}
                </v-list-item-title>
              </v-list-item>
            </v-list>
          </v-menu>
          <div ref="container"></div>
        </div>
      </div>
    </v-app>
  </div>
</template>

<script>
export default {
  data: () => ({
    attachInPlace: true,
    items: ["First", "Second"]
  })
};
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)