Partially revert "healthd: allow custom charger"

* Don't allow custom charger code, prefer to use the existing
  STATIC_HAL_LIBS hook instead.
* Continue to allow devices to replace charger resources, since it's
  just easier to deal with.

This reverts commit 29f7779f.

Change-Id: I34289749c852dd92b7d07ddb3edeb3a1589a7330