Unit Motion - remove m_State

Following D1899/rP22366, we have logic that can handle calling MoveSucceeded() even if we don't go through the special STATE_STOPPING state, and following D1898/rP22365 we no longer need STATE_STOPPING for animation sync either.

m_State can therefore be entirely removed as it is redundant with other information.

Differential Revision: https://code.wildfiregames.com/D1900