cleanup temporary download location when archiving

I noticed a huge amount of used space in /tmp/composer_archiver in my satis machine. 
After digging i found https://github.com/composer/composer/blob/master/src/Composer/Package/Archiver/ArchiveManager.php#L137 
but i couldn't find a place where this is cleaned up after downloading from this temp location.

the vcs downloader removes this target directory prior to downloading anyway and the file downloader uses the cache mechanics and ~/.composer/cache directory additionaly.

there is no use of keeping the downloads in sys_get_temp_dir()/composer_archiver and there is no cleanup strategy yet.
this change deletes the temporary download right after it is used.

ref #2087