Remove unnecessary BlockContentFieldTest

Problem/Motivation

BlockContentFieldTest has the following information in the docblock:

<?php
/**
 * Tests block fieldability.
 *
 * @group block_content
 * @todo Consider removing this test when https://drupal.org/node/1822000 is
 * fixed.
 */
class BlockContentFieldTest extends BlockContentTestBase {
?>

And #1822000: Remove Drupal\field_test\Plugin\Entity\Type\TestEntity in favor of EntityTest already went in.

After asking Berdir, this adds an equivalent test coverage for the link field: #1957748-24: hook_field_prepare_view() is passed empty field values for EntityNG Entities.

Proposed resolution

Remove the testfile

Remaining tasks

None

User interface changes

None

API changes

None