hw/mips/boston: Remove workaround for writes to ROM aborting

Now that the memory system correctly handles writes to ROM for
guest CPUs that may generate exceptions for decode errors, we
can remove the workaround from the boston board.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1513187549-2435-3-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>