core/commands/internal/slice_util: Remove this unused package

The last references to CastToReaders were commented out in 6faeee83
(cmds2/add: temp fix for -r. horrible hack, 2014-11-11) and then
removed completely in 032e9c29 (core/commands2: Updated 'add' command
for new file API, 2014-11-16).

The last references to CastToStrings was removed in a0bd29d5
(core/commands2: Fixed swarm command for new arguments API,
2014-11-18).

core/commands/internal/slice_util.go

-package internal
-
-import (
-	"io"
-
-	u "github.com/ipfs/go-ipfs/util"
-)
-
-func CastToReaders(slice []interface{}) ([]io.Reader, error) {
-	readers := make([]io.Reader, 0)
-	for _, arg := range slice {
-		reader, ok := arg.(io.Reader)
-		if !ok {
-			return nil, u.ErrCast()
-		}
-		readers = append(readers, reader)
-	}
-	return readers, nil
-}
-
-func CastToStrings(slice []interface{}) ([]string, error) {
-	strs := make([]string, 0)
-	for _, maybe := range slice {
-		str, ok := maybe.(string)
-		if !ok {
-			return nil, u.ErrCast()
-		}
-		strs = append(strs, str)
-	}
-	return strs, nil
-}