Disable method replacement in avatar loading

We've seen a significant performance penalty when using
`BatchLoader#__replace_with!`. This defines methods on the batch loader
that proxy to the 'real' object using send. The alternative is
`method_missing`, which is slower.  However, we've noticed that
`method_missing` can be faster if:

1. The objects being loaded have a large interface.
2. We don't call too many methods on the loaded object.

Avatar uploads meet both criteria above, so let's use the newly-released
feature in https://github.com/exAspArk/batch-loader/pull/45.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60903