Untie the engine from mandatory use of configs/mp/ even in singleplayer mode.

Description:

Currently, the engine requires the use of configs/mp/ even in single player mode, causing several issues:

1. Need to duplicate weapon configs in both weapons and mp_ranks folders.
2. Makes story mods cleaner without spaghetti code due to duplicated and potentially incorrect configs. Removing mp would simplify the process.
3. Removing mp speeds up new game load and loading of saves from the main menu.

Proposal:

Untie the engine from mandatory use of configs/mp/ even in single player mode to address the above issues.