MINOR: [C++][Compute] Remove obsolete TODO comment from mode kernel registration

Author: HyukjinKwon

cpp/src/arrow/compute/kernels/aggregate_mode.cc

@@ -495,7 +495,6 @@ void RegisterScalarAggregateMode(FunctionRegistry* registry) {
       NewModeKernel(boolean(), ModeExecutor<StructType, BooleanType>::Exec,
                     ModeExecutorChunked<StructType, BooleanType>::Exec)));
   for (const auto& type : NumericTypes()) {
-    // TODO(wesm):
     DCHECK_OK(func->AddKernel(
         NewModeKernel(type, GenerateNumeric<ModeExecutor, StructType>(*type),
                       GenerateNumeric<ModeExecutorChunked, StructType>(*type))));