libprocessgroup: Remove ACgroupController_getMaxActivationDepth

This was targeted for API 36 which doesn't exist yet, so this was never
released. Now that libprocessgroup does not use the libcgrouprc
interfaces, this can be removed.

Bug: 349105928
Bug: 359609093
Change-Id: I2ea466c9e8bc54bf9178fa74b45e88f26d3e4c69