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.
We should keep this check as we will not want to HLE memory functions when the MMU is enabled.