[cmp.categories.pre] nonequal and nonequivalent are non-used

[StephanTLavavej]

After P1959R0 "Remove std::weak_equality and std::strong_equality" was adopted, N4842 17.11.2.1 [cmp.categories.pre]/1 says:

Each is specified in terms of an exposition-only data member named value whose value typically corresponds to that of an enumerator from one of the following exposition-only enumerations:
enum class eq { equal = 0, equivalent = equal, nonequal = 1, nonequivalent = nonequal }; // exposition only

However, nonequal and nonequivalent are now unused. As they're exposition-only enumerators, they should be editorially removed.