runtime/v2/shim: remove unused SetScore() and remove sys.OOMScoreMaxKillable

The shim.SetScore() utility was no longer used since 7dfc605fc6.

Checking for uses outside of this repository, I found only one external use of
this in gVisor; a9441aea27/pkg/shim/service.go (L262-L264)

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

runtime/v2/shim/util_unix.go

@@ -46,11 +46,6 @@ func getSysProcAttr() *syscall.SysProcAttr {
 	}
 }
 
-// SetScore sets the oom score for a process
-func SetScore(pid int) error {
-	return sys.SetOOMScore(pid, sys.OOMScoreMaxKillable)
-}
-
 // AdjustOOMScore sets the OOM score for the process to the parents OOM score +1
 // to ensure that they parent has a lower* score than the shim
 // if not already at the maximum OOM Score