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
wireprotoserver: remove lock references
AFAICT sshserver.lock is unused. The last caller of it disappeared
in 9f6e0e7ef828 ~18 months ago as part of removing code to support
ancient wire protocol commands. Let's remove some dead code.
Differential Revision: https://phab.mercurial-scm.org/D1986