Remove compute_api_class config option

This option was deprecated in Havana and can now be removed.  Its only
use was for changing out the API class if cells was in use.
Technically, it used to be possible to configure an arbitrary compute
API class.  However, that would have been an unsupported setup, and has
been completely broken since 9f902c7e17249cf5af1504f31d80c0247a02fe20
anyway.

As a part of this change, the cell_type configuration option needed a
default value.  I set it to 'compute'.  My reasoning was that you would
have more compute cells than API cells, so might as well set it to the
more common case.

Change-Id: I5ec44f84267a700da16b350e686eee886e0d1183
Merge "Remove compute_api_class config option"