remove cmake logic for -fconcepts and -fconcepts-ts flags

[nmm0]

I'm suggesting that we remove the logic in our cmake that tries to add the -fconcepts and -fconcepts-ts flags.

First of all, the -fconcepts-ts branch is broken, because the condition checks COMPILER_SUPPORTS_FCONCEPTS instead of COMPILER_SUPPORTS_FCONCEPTS_TS, so it wasn't used anyway.

Secondly, pretty much every modern compiler will enable this if you enable -std=c++20, and we only add this flag if we enable -std=c++20. In the rare cases someone is trying to build mdspan on a platform that doesn't do this, they can simply add -DCMAKE_CXX_FLAGS=-fconcepts to their cmake command line invocation or preset.

Note that us manually adding this flag breaks builds in small niche use cases (that can easily be worked around); i.e. when using CMAKE_CXX_CLANG_TIDY with gcc as clang does not recognize the flag but it is forced on. So the motivation for this is pretty minor but I think it would be good to clean up some of our cmake anyway.

[nmm0]

Also, GCC 15 is removing support for -fconcepts-ts. It won't have any effect anymore.

[mhoemmen]

Thanks @nmm0 ! : - ) config.hpp doesn't seem to use this CMake option anyway, so removing this shouldn't change the code.

[dalg24]

Thanks for cleaning that up