Remove a dead method from lithium-* files.

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
===================================================================
--- src/mips/lithium-mips.cc	(revision 10547)
+++ src/mips/lithium-mips.cc	(working copy)
@@ -581,11 +581,6 @@
 }
 
 
-LRegister* LChunkBuilder::ToOperand(Register reg) {
-  return LRegister::Create(Register::ToAllocationIndex(reg));
-}
-
-
 LUnallocated* LChunkBuilder::ToUnallocated(Register reg) {
   return new LUnallocated(LUnallocated::FIXED_REGISTER,
                           Register::ToAllocationIndex(reg));