Revert "[AMDGPU] Compiler should synthesize private buffer resource descriptor...

Revert "[AMDGPU] Compiler should synthesize private buffer resource descriptor from flat_scratch_init" (#81234)

Reverts llvm/llvm-project#79586

This broke the AMDGPU OpenMP Offload buildbot.
The typical error message was that the GPU attempted to read beyong the
largest legal address.

Error message:
AMDGPU fatal error 1: Received error in queue 0x7f8363f22000:
HSA_STATUS_ERROR_MEMORY_APERTURE_VIOLATION: The agent attempted to
access memory beyond the largest legal address.