Replace call to `_bumpEarningPower` with a call to `super.bumpEarningPower`

[wildmolasses]

The duplicate impl does not seem needed

[funkyenough]

_bumpEarningPower may seem redundant, but we cannot replace it with super.bumpEarningPower or Staker.bumpEarningPower.

bumpEarningPower is marked external in Staker, and both super.bumpEarningPower and Staker.bumpEarningPower resolve to an internal call. Since Solidity doesn't allow internal calls to external functions, it will not compile.

We can either keep the existing _bumpEarningPower pattern or change the parent visibility to public.