Removed IndependentAxes dead zone calculation

Hey there,

I found a strange behavior in thumbsticks values. When pushing a thumbstick toward the min/max of an axis, the other axis is clamped to some extent. This results in thumbsticks being totally inaccurate (at best) or sticked to the min/max of an axis.

Example: when I fully push a thumbstick to the right (0.0f;1.0f), then if I steer slowly up/down, there is a range of about 10° where the values are blocked to (0.0f;1.0f).

The culprit is ```GamePadDeadZone.IndependentAxes```. https://github.com/mono/MonoGame/blob/fcf6e5d5646f8d5e06d02c72c01ed881e2f0563f/MonoGame.Framework/Input/GamePad.XInput.cs#L196
Independently calculating axis dead zone for thumbsticks is not relevant. Thumbsticks should behave like circular inputs even if they are on independent axes driver-wise. Hence, I simplified ```ConvertThumbStick``` to proceed all data as circular.

As far as I checked, it only concerns the XInput / Windows target.