Cleanup: remove unnecessary uses of the bMMU flag

Cleanup; the only functional change is a minor performance optimization for MMIO in bMMU mode.

I'm trying to clarify what exactly bMMU means; as far as I can tell, it means the following:
1. Generate DSI exceptions. (js.memcheck etc.).
2. Generate ISI exceptions.  At least, I think that's the intent; we check bMMU in half a dozen places related to instruction fetch, and there aren't any comments explaining why.
3. Turn off the FakeVMem region.