Refactor: Removed unused vars from Memory.cpp. Also change Disk][ I/O regs to return floating bus.
Fix #232: Mouse was still generating VBl IRQs after being removed.
Merge pull request #238 from AppleWin/memory_refactor

Mainly a tidy-up to remove old, unused code from memory.cpp.

Also:
* Disk][ now returning floating bus for I/O register reads (still a few todo's to check).
* Fixed #232 (mouse VBl IRQs were still occurring after card removed!)