cmake: delete obsolete TODO items [ci skip]

cmake: delete obsolete TODO items [ci skip]

There is always room for improvement, but CMake is up to par now with
autotools, so there is no longer a good reason to keep around these
inline TODO items.

Answering one of questions:

Q: "The gcc command line use neither -g nor any -O options. As a
   developer, I also treasure our configure scripts's --enable-debug
   option that sets a long range of "picky" compiler options."

A: CMake offers the `CMAKE_BUILD_TYPE` variable to control debug info
   and optimization level. E.g.:
   - `Release`    = `-O3` + no debug info
   - `MinSizeRel` = `-Os` + no debug info
   - `Debug`      = `-O0` + debug info

   https://stackoverflow.com/questions/48754619/what-are-cmake-build-type-debug-release-relwithdebinfo-and-minsizerel/59314670#59314670
   https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#default-and-custom-configurations

   For picky warnings we have the `PICKY_COMPILER` options, enabled by
   default.

Closes #12500