fix(VDatatable): remove extra border from expanded functionality

[basselAhmed]

Description

Remove the extra border added to data tables with expanded rows enabled
fixed #6429

Motivation and Context

Fixes an existing issue
#6429

How Has This Been Tested?

visually

Markup:

<template>
  <div id="app">
    <v-app id="inspire">
      <v-container>
        <v-data-table
          :items="desserts"
          class="elevation-1"
          :headers-length="6"
          hide-headers
          hide-actions
          expand
          item-key="name"
        >
          <template v-slot:items="props">
            <tr @click="props.expanded = !props.expanded">
              <td>{{ props.item.name }}</td>
              <td class="text-xs-right">{{ props.item.calories }}</td>
              <td class="text-xs-right">{{ props.item.fat }}</td>
              <td class="text-xs-right">{{ props.item.carbs }}</td>
              <td class="text-xs-right">{{ props.item.protein }}</td>
              <td class="text-xs-right">{{ props.item.iron }}</td>
            </tr>
          </template>
          <template v-slot:expand>
            <v-card flat>
              <v-card-text>Peek-a-boo!</v-card-text>
            </v-card>
          </template>
        </v-data-table>
      </v-container>
    </v-app>
  </div>
</template>

<script>
  export default {
    data: () => ({
      desserts: [
        {
          name: 'Frozen Yogurt',
          calories: 159,
          fat: 6.0,
          carbs: 24,
          protein: 4.0,
          iron: '1%'
        },
        {
          name: 'Ice cream sandwich',
          calories: 237,
          fat: 9.0,
          carbs: 37,
          protein: 4.3,
          iron: '1%'
        },
        {
          name: 'Eclair',
          calories: 262,
          fat: 16.0,
          carbs: 23,
          protein: 6.0,
          iron: '7%'
        },
        {
          name: 'Cupcake',
          calories: 305,
          fat: 3.7,
          carbs: 67,
          protein: 4.3,
          iron: '8%'
        },
        {
          name: 'Gingerbread',
          calories: 356,
          fat: 16.0,
          carbs: 49,
          protein: 3.9,
          iron: '16%'
        },
        {
          name: 'Jelly bean',
          calories: 375,
          fat: 0.0,
          carbs: 94,
          protein: 0.0,
          iron: '0%'
        },
        {
          name: 'Lollipop',
          calories: 392,
          fat: 0.2,
          carbs: 98,
          protein: 0,
          iron: '2%'
        },
        {
          name: 'Honeycomb',
          calories: 408,
          fat: 3.2,
          carbs: 87,
          protein: 6.5,
          iron: '45%'
        },
        {
          name: 'Donut',
          calories: 452,
          fat: 25.0,
          carbs: 51,
          protein: 4.9,
          iron: '22%'
        },
        {
          name: 'KitKat',
          calories: 518,
          fat: 26.0,
          carbs: 65,
          protein: 7,
          iron: '6%'
        }
      ]
    })
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

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)