Remove buggy mapstructure.StringToSliceHookFunc(",").

The `StringToSliceHookFunc(",")` hook is supposed to decode a delim
separated string to a slice of strings. However, the hook only receives
`reflect.Kind` for the target field to determine its type. While it
should be a `[]string`, it can only tell if it's a slice,
thus triggering it for any slice type and failing. This breaks
the unmarshalling of strings to `[]byte` fields.

koanf.go

@@ -252,7 +252,6 @@ func (ko *Koanf) UnmarshalWithConf(path string, o interface{}, c UnmarshalConf)
 		c.DecoderConfig = &mapstructure.DecoderConfig{
 			DecodeHook: mapstructure.ComposeDecodeHookFunc(
 				mapstructure.StringToTimeDurationHookFunc(),
-				mapstructure.StringToSliceHookFunc(","),
 				textUnmarshalerHookFunc()),
 			Metadata:         nil,
 			Result:           o,