Clean up informal language, vague TODOs, and dead code in comments

libeigen/eigen!2191

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>

unsupported/Eigen/src/FFT/fftw_impl.h

@@ -16,8 +16,8 @@ namespace Eigen {
 
 namespace internal {
 
-// FFTW uses non-const arguments
-// so we must use ugly const_cast calls for all the args it uses
+// FFTW uses non-const arguments,
+// so const_cast is needed for all the args it uses.
 //
 // This should be safe as long as
 // 1. we use FFTW_ESTIMATE for all our planning