contributing: Remove shar reference

[emaste]

Summary

Use of shar for submissions has been deprecated for some time.

[emaste]

I'm not sure if modifying the .po file like this is correct or not -- maybe it is supposed to be left alone and handled by translation tooling later?

[kevans]

OK for the content change

[fuz]

Looks reasonable. Porter's Handbook has language explicitly pointing out that we no longer accept ports submitted in shar format. Maybe we should add a similar disclaimer here?