test i2c asynch: Remove printf usage from interrupt context

The cbmaster_done function is a callback which will be called from
the asynch I2C interrupt handler. Calling to printf from this context
sometimes lead to missing interrupts on the slave side. This was at least
encountered on STM32F3 MCUs.

features/unsupported/tests/mbed/i2c_master_slave_asynch/main.cpp

@@ -45,7 +45,6 @@ I2CSlave slave(D3, D6);
 volatile int why;
 volatile bool master_complete = false;
 void cbmaster_done(int event) {
-    printf("cbmaster_done\n");
     master_complete = true;
     why = event;
 }