[mips][NFC] Remove old FIXME comment

This was fixed in rL229595 but this comment was missed.

Differential Revision: https://reviews.llvm.org/D70231

llvm/lib/Target/Mips/MCTargetDesc/MipsMCTargetDesc.cpp

@@ -44,7 +44,6 @@ using namespace llvm;
 #include "MipsGenRegisterInfo.inc"
 
 /// Select the Mips CPU for the given triple and cpu name.
-/// FIXME: Merge with the copy in MipsSubtarget.cpp
 StringRef MIPS_MC::selectMipsCPU(const Triple &TT, StringRef CPU) {
   if (CPU.empty() || CPU == "generic") {
     if (TT.getSubArch() == llvm::Triple::MipsSubArch_r6) {