boards native_sim_64: Remove hwmv1 compatible board definition

To use this functionality one must now use the native_sim//64
(variant) version. The old hwmv1 compatible name for the board
was left provisionally while all tests in tree were ported.
That being now done, this old name can be removed.

After this change one cannot build anymore
targeting native_sim_64, but must instead build
targeting native_sim/[native]/64.
For twister tests the old name is already not valid.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>