STM32 16 bits tickers: save useless instance init

TimMasterHandle.Instance initialization can be removed from here,
because it will either have been already done previously,
or it will be done in the us_ticker_init() call immediately below.

targets/TARGET_STM/us_ticker_16b.c

@@ -49,8 +49,6 @@ void us_ticker_init(void)
 
 uint32_t us_ticker_read()
 {
-    TimMasterHandle.Instance = TIM_MST;
-
     if (!us_ticker_inited) us_ticker_init();
 
     uint16_t cntH_old, cntH, cntL;