Remove old help_* language files

https://github.com/phpbb/phpbb/pull/3613 just added the new files with keys. For easier diff generation on the update the original files have been kept.

• We shall fix all stuff in the old help_*.
• Close the package from language changes.
• Create the diff for translators.
• Run the phpBB/develop/lang_migrate_help_lang.php to generate the new key based files
• Remove the old file (I'd even do that after the update is out (first minor or something), so translators that don't read our posts correctly don't have too much trouble finding the changes).