sim: mips: delete mmu stubs to move to common sim_{read,write}

The only unique thing about mip's sim_{read,write} helpers is the call to
address_translation on the incoming address.  When we look closer at that
function though, we see it's just a stub that maps physical to virtual,
and the cache/return values are hardcoded.  If we delete this function,
we can then collapse all the callers and drop the custom sim_{read,write}
logic entirely.

Some day we might want to add MMU support, but when we do, we'll want to
have the common layers handle things so all targets benefit.