Remove a redundant option_map and some factorization

[nobrakal]

Hi there,

This small PR remove a redundant definition of option_map. While changing the file, I was not able to retain myself to factorize also some code; I hope this is ok !

A little bit of self-promotion, this redundancy was detected while testing an (experimental) library called asak.

[vouillon]

Thanks for your pull request! Option.map is a fairly recent addition to the OCaml standard library, so we prefer to wait a bit before merging it.