fix(VDataTable): remove unnecessary colspan in mobile view

[jacekkarczmarczyk]

Motivation and Context

colspan just seems unnecessary in mobile mode

How Has This Been Tested?

visually, updated snapshots

Markup:

<template>
  <v-container>
    <v-data-table
      :headers="headers"
      :items="desserts"
      :expanded.sync="expanded"
      show-select
      item-key="name"
      show-expand
      class="elevation-1"
    >
      <template v-slot:expanded-item="{ headers }">
        <td>
          <div style="width: 200px; height: 10px; background: red" />
        </td>
      </template>
    </v-data-table>
  </v-container>
</template>

<script>
  export default {
    data () {
      return {
        expanded: [],
        headers: [
          {
            text: 'Dessert (100g serving)',
            align: 'left',
            sortable: false,
            value: 'name',
          },
          { text: 'Calories', value: 'calories' },
          { text: 'Fat (g)', value: 'fat' },
        ],
        desserts: [
          {
            name: 'Frozen Yogurt',
            calories: 159,
            fat: 6.0,
          },
          {
            name: 'Ice cream sandwich',
            calories: 237,
            fat: 9.0,
          },
        ],
      }
    },
  }
</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 breaking 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)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)