clang-tidy9.0.0 added a new check: bugprone-branch-clone
(https://releases.llvm.org/9.0.0/tools/clang/tools/extra/docs/ReleaseNotes.html)
This doesn't like both branches of an if are the same.
This lead to a warning in rainbow, as where the maximum of two values (which
are always the same) is computed in a macro.
I don't always agree with this warning, but here I think it's worth to
remove the macro.