vmstat - Remove the busy_time == 0 hack

vmstat - Remove the busy_time == 0 hack

Remove a very old busy_time == 0 hack which assumed that a delta busy time
of 0 with transactions present simply meant that a transaction didn't
complete within one second and that the device was 100% busy.

In fact, disk devices are so fast these days (particularly SSDs) that it is
possible for many transactions to complete without causing the busy counter
to tick-over.  The device is more likely to be less than 1% busy.

This fixes the '% busy' display for e.g. 'systat -vm 1' to properly report
0% in these situations.