wireprotoserver: remove sshserver.getarg()
AFAICT the last consumer of this helper method was removed by
d054cc5c7737 in 2010.
Differential Revision: https://phab.mercurial-scm.org/D1987
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program