revert r273728 and parts of r306589, iicbus no-stop by default feature

Since drm2 removal, there has not been any consumer of the feature in the
tree. I am also unaware of any out-of-tree consumer.
More importantly, the feature has been broken from the very start, both
before and after r306589, because the ivar was set on a device that does
not support it and it was read from another device that also does not
support it.

A bus-wide no-stop flag cannot be implemented as an ivar as iicbus
attaches as a child of various drivers. Implementing the ivar in each
and every I2C driver is just impractical.

If we ever want to implement this feature properly, then probably the
easiest way to do it would be via a flag in the softc of iicbus.
In fact, we might have to do that in the stable branches if we want to
fix the code for them.

Reported by: ian (long time ago)
MFC after: 1 month (maybe)
X-MFC-note: cannot just merge the change, must keep drm2 happy