Stop generating Contributor:/Maintainer: for new device ports

Contributor:

The Git history is a much better representation of contributors to a particular device port. Also see: pmaports#543 (closed)

Maintainer:

Most device ports that are added to pmaports (particularly the testing category) are not actually "maintained". Many of them are never updated after the initial contribution. There is little reason to list a "Maintainer:" if the device package is actually not actively maintained by that person.

Let's stop generating the Maintainer: line by default. Instead, contributors should add the lines themselves if they are willing to (actively) maintain the device package for a longer period of time. Once they no longer want to maintain the package, the Maintainer: line should be removed again.