Remove obsolete retsigtype

Remove obsolete retsigtype

Only relevent for pre-c89 K&R compilers.

[mmeffie@sinenomine.net: avoid changes to src/external]

Change-Id: I1b3bf14ddd50f1a6b3d50e0376abffffdb64fb81
Reviewed-on: https://gerrit.openafs.org/12203
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>