changed mutable instances in MdtDriftCircleOnTrack.h

Changed the declaration of m_localAngle, m_positionAlongWire and m_driftTime from "mutable float" to "float" to avoid potential problems for AthenaMT comming from instances of mutable members. Since the mutable only has impact on the compilation process but will not change the resulting code, it is good enough that the code still compiles.