CurieMailbox: remove HW access code which is now in cores/arduino#549

eriknyquist wants to merge 1 commit intoarduino:codk-m-corelibsarduino/ArduinoCore-arc32:codk-m-corelibsfrom

eriknyquist:mailbox_coreeriknyquist/corelibs-arduino101:mailbox_coreCopy head branch name to clipboard