firmware: unused and uncaught MEASURE_SILENT

Note to myself

The measureTxMode may be set to MEASURE_SILENT in order not to send measurement packets even during an active frame and established synchronization. There are even two setter functions measure_set_tx_active() and measure_set_tx_silent().

However, this possibility is used nowhere. In the current implementation there is even a bug in the measurement scheduler: The packet transmission is started without checking the measureTXMode. This may lead to situations where the scheduler tries to start the transmission of a packet that never was prepared before (The frame preparation respects the measureTxMode).

TLDR: Maybe this MEASURE_SILENT state and all the corresponding functionality should just be removed?