[RISCV] Remove getOffsetOfLocalArea() (#93765)

For RISC-V, it's always 0 and I don't see any reason we will
change it in the future.

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -878,9 +878,9 @@ RISCVFrameLowering::getFrameIndexReference(const MachineFunction &MF, int FI,
           StackID == TargetStackID::ScalableVector) &&
          "Unexpected stack ID for the frame object.");
   if (StackID == TargetStackID::Default) {
-    Offset =
-        StackOffset::getFixed(MFI.getObjectOffset(FI) - getOffsetOfLocalArea() +
-                              MFI.getOffsetAdjustment());
+    assert(getOffsetOfLocalArea() == 0 && "LocalAreaOffset is not 0!");
+    Offset = StackOffset::getFixed(MFI.getObjectOffset(FI) +
+                                   MFI.getOffsetAdjustment());
   } else if (StackID == TargetStackID::ScalableVector) {
     Offset = StackOffset::getScalable(MFI.getObjectOffset(FI));
   }