AMDGPU: Remove note about spill handling changes

This was reverted from the release by
8ff2643

Author: arsenm

llvm/docs/ReleaseNotes.rst

@@ -152,9 +152,6 @@ Changes to the AMDGPU Backend
   improves the interaction between AMDGPU buffer operations and the LLVM memory
   model, and so the non `.ptr` intrinsics are deprecated.
 
-* SGPR spilling is now performed to virtual VGPRs. This should avoid
-  some assorted register allocation failures.
-
 * Backend now performs range merging of "amdgpu-waves-per-eu" attribute based on
   known callers.