[4.3] Remove outer navigation map read locks

[smix8]

Removes outer navigation map read locks as we apparently go where no one knows what will happen when we dare to lock twice.

Fixes #101318 for Godot 4.3.

Not relevant for Godot 4.2. as that was happy-thread-crash-land without those locks and also not relevant for Godot 4.4 as that has this entire code changed and works with a single map iteration lock.

[akien-mga]

For the record, this was changed in 4.4 with d51615b.

There's a few more RWLockRead read_lock(map_rwlock); in the file, but I assume the fix here is to just avoiding calling it twice in functions which then call get_closest_point_info?

[smix8]

Yes, the other locks need to stay, else we crash again with user pathfinding threads like we did in Godot 4.2 when the map changes and invalidates all the pointers. This only removes the outer function locks because they all call the internal get_closest_point_info() function that also read locks the same lock.

[akien-mga]

Thanks!