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

[eriknyquist]

The mailbox HW initialisation now happens in cores/arduino/main.cpp.
All of the core code for accessing the mailbox HW now lives in
cores/arduino/mailbox.cpp; the CurieMailbox library is now just a wrapper
that calls the methods from mailbox.cpp, plus a ring buffer.

NOTE: this must not be merged until #547 has been merged into master, and also migrated over to the target branch of this PR (codk-m-corelibs)

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