Remove dead code

Verified with

```
deadcode -test ./...
```

Followed-up with a lint check.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/pkg/container/util.go

@@ -48,11 +48,6 @@ func ListSources() ([]string, error) {
 	return ret, nil
 }
 
-func DoesContainerExists(name string) bool {
-	fullPath := ImageFile(name)
-	return util.IsFile(fullPath)
-}
-
 func DoesSourceExist(name string) bool {
 	fullPath := RootFsDir(name)
 	return util.IsDir(fullPath)