downloads: Remove unexistent method ephy_download_pause()
downloads: Remove unused method ephy_download_new()
downloads: Remove ephy_download_set_auto_destination()

It's currently no-op.
downloads: Remove ephy_download_start()

This doesn't make sense now in WebKit2, because a WebKitDownload object
represents an already started download.
downloads: Remove unused EphyDownload:source property
downloads: Rename ephy_download_new_for_download as ephy_download_new