pkg/signal: remove stubs for deprecated package
pkg/signal: remove stubs for deprecated package

The pkg/signal package was moved to github.com/moby/sys/signal in
28409ca6c76030ef1cd277cf73223a47cf3f1dbe. The DefaultStopSignal const was
deprecated in e53f65a9168aaf4289a490bd56d9e05b46c08bec, and the DumpStacks
function was moved to pkg/stack in ea5c94cdb94f74808958732d8b3254921f52be4f,
all of which are included in the 22.x release, so we can safely remove these
from master.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

thaJeztah/docker,Mokolea/docker,konstruktoid/docker-upstream,mahak/docker,moul/docker,moul/docker,mahak/docker,jimjag/docker,moby/moby,thaJeztah/docker,Mokolea/docker,jimjag/docker,mahak/docker,konstruktoid/docker-upstream,konstruktoid/docker-upstream,trebonian/docker,moby/moby,Mokolea/docker,moby/moby,trebonian/docker,cpuguy83/docker,trebonian/docker,Ramzec/docker,trebonian/docker,cpuguy83/docker,konstruktoid/docker-upstream,moby/moby,Ramzec/docker,mahak/docker,Mokolea/docker,jimjag/docker,cpuguy83/docker,Ramzec/docker,Ramzec/docker,cpuguy83/docker,moul/docker,thaJeztah/docker,moul/docker,jimjag/docker,thaJeztah/docker