Remove dead pre-Contao-5.0 picker classes

TreeSelect and FileSelect are marked "unusable since Contao 5.0" and call BackendUser::authenticate(), which was removed in Contao 5.0, so they fatal on the minimum supported Contao (^5.7). They have no references anywhere. Removed as part of the 2.5 baseline (Contao ^5.7 / PHP ^8.4).

Author: zonky2