SCUMM: Remove unsued method loadThumbnailFromSlot(int slot).
SCUMM: Slight cleanup in savegame code.
SCUMM: Slight cleanup in detection.cpp.
SCUMM: Cleanup querySaveMetaInfos implementation.

This results in less code and also less I/O operations (including seeking).
SCUMM: Small formatting fix.
GRAPHICS: Fix SharedPtrSurfaceDeleter for nullptr inputs.