Revert "Let precise things and put some references"

This reverts commit fccf6cfdee7b63feb6deb96e9c8cd398c20b44a3.
Revert "Resolves: fdo#72174 convert_add from "at" or "atm" to "Pa" gives same answer"

This reverts commit c2dd0ef4b891a3d126dbe70ab5b76016b69d491c.

OpenFormula defined CONVERT after Excel that treats "at" and "atm" as
equivalent. Let's not come up with different factors here, even if
academically these two units c/would be different things.