Removed REQUIREMENT_INFO/REQUIREMENT_OK from most hook_requirements() items

Most entries returned by hook_requirements() no longer return an enforced severity for positive assertions.

1. All neutral/informational requirements check results should not return a hard-coded severity level.
2. Only return REQUIREMENT_INFO or REQUIREMENT_OK if you want to enforce the corresponding visual presentation.

This is because:

• The Drupal installer defaults to REQUIREMENT_OK for positive assertions, so the user who is installing Drupal gets the appropriate visual feedback for met requirements.
• The Status report page defaults to REQUIREMENT_INFO for all requirements checks. The visual feedback for positive assertions would turn the entire page "green" otherwise.

Drupal 7

function mymodule_requirements() {
  $exists = (...);
  $requirements['foo'] = array(
    'title' => 'Foo extension',
    'value' => $exists ? $t('Enabled') : $t('Missing'),
    'severity' => !$exists ? REQUIREMENT_ERROR : REQUIREMENT_OK,
  );
  return $requirements;
}

Drupal 8

function mymodule_requirements() {
  $exists = (...);
  $requirements['foo'] = array(
    'title' => 'Foo extension',
    'value' => $exists ? $t('Enabled') : $('Missing'),
    // Note: Depending on your code logic, you can also omit the 'severity' key entirely.
    'severity' => !$exists ? REQUIREMENT_ERROR : NULL,
  );
  return $requirements;
}