Better cleanup when deleting Paragraph Types

[laryn]

I'm finding that deleting a Paragraph Type doesn't fully clean up -- the type and the field instances are removed, but if there are fields that were only used in that type, they remain in the configuration as orphans.

Right now we're manually deleting paragraphs field instances when deleting a paragraphs type (in paragraphs_bundle_delete):

• paragraphs/paragraphs.module

  Lines 1139 to 1142 in e827b5a

  	$sub_names = config_get_names_with_prefix('field.instance.paragraphs_item.' . $bundle->bundle . '.');
  	foreach ($sub_names as $sub_name) {
  	config($sub_name)->delete();
  	}

When deleting the field instances, I think we should instead use field_instance_delete which will also give us an option to set the $field_cleanup parameter to TRUE and also delete the field itself if it is no longer in use:

• https://github.com/backdrop/backdrop/blob/5430d0a5ad9858d4caa76275764e66de2daeb3b9/core/modules/field/field.crud.inc#L860-L884

[laryn]

It looks like a minor reorganization of code may be enough here. (Field cleanup defaults to TRUE in that function).