[AMDGPU] Switch to the new cl option amdgpu-atomic-optimizer-strategy.

Atomic optimizer is turned on by default through D152649. This patch
removes the usage of old command line option amdgpu-atomic-optimizations
and transfer the responsibility to `amdgpu-atomic-optimizer-strategy`.

We can safely remove old option when LLPC remove its all usage.

Reviewed By: foad, arsenm, #amdgpu, cdevadas

Differential Revision: https://reviews.llvm.org/D153007