[CodeGen] Remove redundant declaration createGreedyRegisterAllocator (NFC)

The function is declared in llvm/include/llvm/CodeGen/Passes.h.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -180,10 +180,6 @@ FunctionPass* llvm::createGreedyRegisterAllocator() {
   return new RAGreedy();
 }
 
-namespace llvm {
-FunctionPass *createGreedyRegisterAllocator(RegClassFilterFunc Ftor);
-}
-
 FunctionPass *llvm::createGreedyRegisterAllocator(RegClassFilterFunc Ftor) {
   return new RAGreedy(Ftor);
 }