Remove header for option modules (#6514)

Remove header for option modules (#6514)

This PR further simplifies the option declaration by removing the header attribute from module options.
Instead of specifying it manually, it is now automatically generated from the filename of the toml file. The header files and the toml files use matching names already, so this PR simply removes another mechanism that is not used anyway.
This PR also does a minor cleanup of the Options class in the mkoptions.py script.