[AMDGPU] Remove getMaxWavesPerCU in favour of getWavesPerWorkGroup.

Summary:
These methods were identical. I chose to remove getMaxWavesPerCU because
I think Max in the name was misleading. NFC.

Reviewers: arsenm, rampitec

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76859