chore(library/init/logic): remove an unnecessary case split from `or.by_cases` (#755)
chore(library/init/logic): remove an unnecessary case split from `or.by_cases` (#755)

`if h : x then f else false.elim _` is the same as `f`.
This means we can drop the `decidable q` argument.

A docstring is also added, but the behavior it describes has not changed.



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