scripts: remove boards_legacy sub-folder from list_boards.py

Fixes: #69785

The boards_legacy sub-folder was temporarily introduce in collab-hwm
branch during porting to HWMv2.

This should have been removed before merging collab-hwm to main as it
prevent looking up boards in oot roots.

Removing the temporary sub-folder for HWMv2.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>