pkg/signal: remove stubs for deprecated package

The pkg/signal package was moved to github.com/moby/sys/signal in
28409ca. The DefaultStopSignal const was
deprecated in e53f65a, and the DumpStacks
function was moved to pkg/stack in ea5c94c,
all of which are included in the 22.x release, so we can safely remove these
from master.

Signed-off-by: Sebastiaan van Stijn <[email protected]>