MOUNT to use d81attach sticky option instead of "mount memory"

[dansanderson]

After MEGA65/mega65-core#863 is implemented in Hyppo, replace the feature where the MOUNT command stores its last argument and remounts on reset (see b65.src:remount) to use the Hyppo feature instead. This will make the logic that decides the default mount easier for the user to understand, and still provide similar functionality (Intro Disk, development workflow).