Clean up tbx-for-filesystem-import directory

On a client instance the tbx-for-filesystem-import folder consumes 5.6 GB.

The several termcollection directories contain multiple versions of a TBX file, for example in one:

Die dortigen Verzeichnisse sind voll mit EInträgen wie:
rw-rr- 1 www-data www-data 7153659 Jul 25 06:13 100-phpDTcKjI
rw-rr- 1 www-data www-data 7153659 Jul 26 06:11 101-phpBdqO55
rw-rr- 1 www-data www-data 7155258 Jul 29 06:12 102-php3hpfGm
rw-rr- 1 www-data www-data 7175315 Jul 30 06:17 103-phpIvYdnR
rw-rr- 1 www-data www-data 7178402 Jul 31 06:13 104-php7XLpmF
rw-rr- 1 www-data www-data 7178403 Aug 1 06:11 105-phpYBdc2G
rw-rr- 1 www-data www-data 7178403 Aug 2 06:12 106-phpo1QPVm
rw-rr- 1 www-data www-data 7178403 Aug 5 06:12 107-phpbxrwDm
rw-rr- 1 www-data www-data 7178403 Aug 6 06:06 108-phpHkPqEN
rw-rr- 1 www-data www-data 7178403 Aug 7 06:06 109-phpAYiu0S
rw-rr- 1 www-data www-data 6430442 Mar 22 06:20 10-phplWAj3X
rw-rr- 1 www-data www-data 7180845 Aug 8 06:08 110-phpUNnkay
rw-rr- 1 www-data www-data 7180845 Aug 9 06:06 111-phpOgTWnU
rw-rr- 1 www-data www-data 7180845 Aug 12 06:09 112-phppT6dCc
rw-rr- 1 www-data www-data 7153659 Jul 25 06:13 100-phpDTcKjI
rw-rr- 1 www-data www-data 7153659 Jul 26 06:11 101-phpBdqO55

This files are produced by the saveFileLocal Methode in der Tbx.php

What we need: A fix the files are cleaned up on import, and a fix script which ckeans up the old unneeded data.

When tbx file is imported via api or frontend, and when the 'remove older terms than import' is checked or the 'remove old terms date' is given, all tbx files in the tbx-import directory for this term collections with modify date older or equal to the given date will be removed.