[Gameplay] - Remove min distance from siege towers

While it may be historically correct, it arguably can lead to very broken looking behavior where siege towers try to get out of the min range, but fail since the are attacked and end up shifting and turning around uselessly, which just looks broken.

And since the min distance for siege towers is very small, I doubt that it will actually change anything about the gamplay.

accepted by : @chrstgtr

Differential revision: https://code.wildfiregames.com/D4657