Remove duplicate CMake in modules implementation

[davidstone]

It looks like two different approaches to the same problem got committed to this file. Remove the more complicated approach and keep the simpler approach. This fixes CMake warnings about the same source file appearing multiple times.

Pre-merge checklist

• I've read [CONTRIBUTING.md]
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp ([how-to])
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md