[ticket/12726] PSR2.Namespaces.NamespaceDeclaration

PHPBB3-12726
[ticket/12726] Ensure PHP files do not contain any unused use statements

PHPBB3-12726
[ticket/12726] Fix coding style

PHPBB3-12726
[ticket/12726] Update error message

PHPBB3-12726
[ticket/12726] Fix coding style

PHPBB3-12726
Merge pull request #2625 from Nicofuma/ticket/12726

[ticket/12726] Add Code Sniffer sniff ensuring PHP files do not contain any unused use statements

* Nicofuma/ticket/12726:
  [ticket/12726] Fix coding style
  [ticket/12726] Update error message
  [ticket/12726] Fix coding style
  [ticket/12726] Ensure PHP files do not contain any unused use statements
  [ticket/12726] PSR2.Namespaces.NamespaceDeclaration