[wpimath] Remove or deprecate unused WheelPositions interface/concept

[KangarooKoala]

#6673 migrated Kinematics, Odometry, and PoseEstimator to not use the WheelPositions interface (Java)/concept (C++). The only remaining uses are as superinterfaces of DifferentialDriveWheelPositions and MecanumDriveWheelPositions. We probably could completely remove the interface/concept since it should only impact kinematics/odometry/pose estimator implementations for custom drivetrain types who would be impacted by #6673 already, but I can also see the case for deprecating first for a year.

Once we decide which of removing and deprecating we want to do, this would be a great first PR for somebody looking to start contributing to WPILib!