docs: Removing obsolete ActionBar training

The action bar training classes are being superseded by the new App
Bar training, which uses the appcompat Toolbar. This CL removes the
obsolete docs. The new docs are covered by CL http://ag/730414 ; both
CLs will go live together. (There's a separate CL, http://ag/732803 ,
setting up redirects from obsolete pages to the new ones.)

There are a couple of topics in the old Action Bar API Guide which
are not yet covered by training, so this CL does *not* remove that
page--that'll happen when I add App Bar docs covering those areas.

Also changed links/references to the now-deleted pages where I saw
them--and while I had the files opened, changed instructions that
had told devs to use (now-deprecated) ActionBarActivity, and told
them instead to use AppCompatActivity.

See first comment for doc stage location.

Change-Id: Id548d8f483d85f2eef9f744d258ed113554eb6fc