Duke3D: Remove the 'previousboardfilename' mapart hack

This was a bandaid that prevented repeatedly loading the same mapart when loading savegames on the same map. This meant having to constantly update the previousboardfilename buffer whenever clearing mapart, which is undesirable.

It existed mainly to prevent a crash related to tile caching, which has since been resolved. It also reduces memory usage when repeatedly loading the same save with mapart -- but this didn't really fix the fundamental issue, as memory usage could still increase when switching between two maps (e.g. with mapstate).

Fixes mapart not reloading after quitting to title (or ending an episode) and then loading a savegame. Fixes #224 (closed)