[examples] Remove unused variable 'BB' in BrainF.cpp (NFC)

/Users/jiefu/llvm-project/llvm/examples/BrainF/BrainF.cpp:92:15: error: unused variable 'BB' [-Werror,-Wunused-variable]
  BasicBlock* BB = builder->GetInsertBlock();
              ^
1 error generated.

llvm/examples/BrainF/BrainF.cpp

@@ -89,7 +89,6 @@ void BrainF::header(LLVMContext& C) {
 
   //%arr = malloc i8, i32 %d
   ConstantInt *val_mem = ConstantInt::get(C, APInt(32, memtotal));
-  BasicBlock* BB = builder->GetInsertBlock();
   Type* IntPtrTy = IntegerType::getInt32Ty(C);
   Type* Int8Ty = IntegerType::getInt8Ty(C);
   Constant* allocsize = ConstantExpr::getSizeOf(Int8Ty);