ES.74 is redundant with ES.6

[tituswinters]

ES.6: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Res-cond
ES.74: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Res-for-init

I'd strike ES.74 and merge it's notes and enforcement details into ES.6.

[AndrewPardoe]

Jonathan, could you please prepare a PR?