[ticket/11605] Use empty_dir to better delete files and dirs of extensions

PHPBB3-11605
[ticket/11605] Remove unused copied_files property

PHPBB3-11605
Merge remote-tracking branch 'nickvergessen/ticket/11605' into develop

* nickvergessen/ticket/11605:
  [ticket/11605] Remove unused copied_files property
  [ticket/11605] Use empty_dir to better delete files and dirs of extensions