Remove mention of API counter type.
Remove mention of API counter type.

Reasons:

* It behaves unlike agent check counter and count types, and unlike
dogstatsd counter type, which confuses people.
* We want to discourage its use because it behaves badly when the
counter value gets reset over a rollup interval.
* Its documentation has been wrong for a long time without anyone
seeming to notice, which implies people won't miss its absence.

jhotta/documentation,jhotta/documentation,jhotta/documentation,jhotta/documentation,jhotta/documentation,jhotta/documentation