[AMDGPU] Reconsider MIOpen softmax once it is fixed

[pxl-th]

If this issue is fixed we can add it back:
ROCm/MIOpen#2966

Removed in #585

[CAHEK7]

@pxl-th can you check that the latest MIOpen develop works fine for you?
You have to remove performance database from the previous runs or test it on a clean system.

ROCm/MIOpen#2966 is closed.