Looks like some cruft has been left over from previous refactorings.
This removes dead variables, which should not have side effects in their
constructors. In cases where the variable initialization has a purpose
[[maybe_unused]] is inserted to silence compiler warnings.
This reduces the amount of boilerplate. More importantly, it provides
a place to add compiler flags (such as -O3) without having to add it
to every subproject (and the risk of forgetting to include it).
