Its intent was to show which headers are expected to be useable by
Catch2's users, and to enforce their inclusion in the single header
distribution at the right place.
Given the new library model, the second use case is not needed and
the first one is better served with documentation and physical file
layout.
Now that Catch2 is a proper library, we can always build the full
library (comparatively minor slowdown) and the user can avoid
including benchmarking headers to avoid the compilation slowdown.
