Use unix.SignalNum in ParseSignal on unix platform

This removes the signalMap on unix platform, since
the signalMap on different architectures is not same,
especially it's wrong on mipsx.

golang.org/x/sys/unix now has a SignalNum func to convert
signal name to a number, thus there's no need to keep
this redundant map.

Windows platform still needs to have a signalMap, since
golang.org/x/sys/windows doesn't have corresponding
functions.

Address: #3061

Signed-off-by: Shengjing Zhu <zhsj@debian.org>

signals.go

@@ -20,13 +20,11 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"strconv"
-	"strings"
 	"syscall"
 
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/images"
-	"github.com/opencontainers/image-spec/specs-go/v1"
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 // StopSignalLabel is a well-known containerd label for storing the stop
@@ -83,23 +81,3 @@ func GetOCIStopSignal(ctx context.Context, image Image, defaultSignal string) (s
 
 	return config.StopSignal, nil
 }
-
-// ParseSignal parses a given string into a syscall.Signal
-// it checks that the signal exists in the platform-appropriate signalMap
-func ParseSignal(rawSignal string) (syscall.Signal, error) {
-	s, err := strconv.Atoi(rawSignal)
-	if err == nil {
-		sig := syscall.Signal(s)
-		for _, msig := range signalMap {
-			if sig == msig {
-				return sig, nil
-			}
-		}
-		return -1, fmt.Errorf("unknown signal %q", rawSignal)
-	}
-	signal, ok := signalMap[strings.TrimPrefix(strings.ToUpper(rawSignal), "SIG")]
-	if !ok {
-		return -1, fmt.Errorf("unknown signal %q", rawSignal)
-	}
-	return signal, nil
-}