remove deprecated argument names

[Enchufa2]

In the next release of units, arguments x andn y in ud_are_convertible() are deprecated, and renamed as from and to. x and y were unfortunate names. This fix simply removes the old name, so that this works regardless of the units version. But there is no rush to push an update to CRAN, because we still support x and y with a warning

[cdmuir]

Thank you!