[ticket/12390] Remove documentation, tests and other files from vendor/

PHPBB3-12390
[ticket/12390] Move cleaning of vendor into it's own target

PHPBB3-12390
[ticket/12390] Add comment to composer.json to update the build target

PHPBB3-12390
Merge pull request #2299 from nickvergessen/ticket/12390

[ticket/12390] Remove documentation, tests and other files from vendor/

* nickvergessen/ticket/12390:
  [ticket/12390] Add comment to composer.json to update the build target
  [ticket/12390] Move cleaning of vendor into it's own target
  [ticket/12390] Remove documentation, tests and other files from vendor/