[Various] obsoleting the annotate level 2 interface

[Jim Blandy]

I think we can expect MI to continue to change over time.  It's a new
interface, and we're all still learning what we need from it, and how
it should behave.  To expect complete stability at this point is to
ensure that less-than-helpful behaviors also remain "stable".

While GDB developers should obviously take care when making MI-visible
changes (and I think they do), MI's clients also need to be tolerant
of the changes that will occur, and they need to be constructive ---
for example, by contributing test cases and good bug reports.