d6 block description migrated into d8 block title for custom blocks

Steps to reproduce:
- Create a custom block on d6 at /admin/build/block/add
- Add 'Block Description' in the Block Description field.
- Add 'Block Title' in the Block Title field

Expected behavior:
In the D8 site, the block should have 'Block Title' set as the Title field.

Actual behavior:
'Block Description' was set as the Title field instead of 'Block Title'.