fix(VCalendar): remove background for week view mode

[SonTT19]

Description

fixes #18988
regression 4820347

Markup:

<template>
  <v-app theme="dark">
    <v-container>
      <v-calendar v-model="msg" view-mode="week" :events="events"></v-calendar>
    </v-container>
  </v-app>
</template>

<script setup>
  import { ref } from 'vue'

  const events = [
    {
      title: 'First',
      start: new Date('2024-01-22T02:00:00Z'),
      end: new Date('2024-01-22T04:30:00Z'),
      color: 'blue',
    },
    {
      title: 'Second',
      start: new Date('2024-01-23T05:30:00Z'),
      end: new Date('2024-01-23T08:30:00Z'),
      color: 'red',
    },
    {
      title: 'Third',
      start: new Date('2024-01-23T08:30:00Z'),
      end: new Date('2024-01-23T09:30:00Z'),
      color: 'green',
    },
    {
      title: 'Fourth',
      start: new Date('2024-01-24T02:01:00Z'),
      end: new Date('2024-01-24T04:01:00Z'),
      color: 'green',
    },
  ]

  const msg = ref([new Date('2024-01-23T14:30:00Z')])
</script>