Review the need for `filter_sideload_move_temp_file_to_s3()` with WordPress 4.4

The according to the documentation, the `filter_sideload_move_temp_file_to_s3()` method of `S3_Uploads` exists to work around [Core bug #29257](https://core.trac.wordpress.org/ticket/29257).

This issue is marked as fixed for 4.4, so we should review whether we still need this method in the plugin. If we end up removing the plugin's method, we should put a check for the WordPress version requirement in place, and document this, similar to #69.

Start by inspecting S3_Uploads::filter_sideload_move_temp_file_to_s3() and the WordPress Core bug #29257, then compare the handling described in issue #69. Determine whether WordPress 4.4 makes the method unnecessary; done means a documented removal decision, with a WordPress version check if the method is removed.

Written by the indexing model from the issue text.