EmulatorPkg/DxeTimerLib: drop superfluous cast
    
    "gTimerEvent" has type EFI_EVENT already, drop the superfluous cast.
    
    Cc: Andrew Fish <afish@apple.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Ray Ni <ray.ni@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>