Compatibility to new Teensyduino toolchain (gcc 11.3)

To be compatible to the new gcc 11.3 toolchain I needed to remove the frequency uitility (timers accepted 3.45_MHz , 17kHz etc as period parameter). The corresponding header (frequency.h) was a wild hack which is not accepted anymore by the new gcc. Lets hope that future implementations of allow frequency types in addition to duration types.