Revert "hw/mips: MIPS Boston board support"

This reverts commit d3473e147a754e999718bf6fcb015d9978c6a1ee.

This commit creates a board which defaults to having 2GB of RAM.
Unfortunately on 32-bit hosts we can't create boards with 2GB of RAM,
and so 'make check' fails. I missed this during testing of the
merge, unfortunately. Luckily the offending commit is the last
one in the merge request, so we can just revert it for now.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
