[ticket/12938] Remove the 'Download all attachments' feature.

PHPBB3-12938
[ticket/12938] Remove functional tests for 'Download all attachments'.

PHPBB3-12938
Merge pull request #2852 from bantu/ticket/12938

[ticket/12938] Remove the 'Download all attachments' feature.

* bantu/ticket/12938:
  [ticket/12938] Remove functional tests for 'Download all attachments'.
  [ticket/12938] Remove the 'Download all attachments' feature.