Flipgrid fixed an issue on their end that required us to make the iframe
for Flipgrid embeds taller than the actual content. This commit removes
the custom aspect ratio so that these videos will use the 16:9 default.

Tested by pasting this link into an annotation: https://flipgrid.com/s/030475b8ceff

Slack thread: https://hypothes-is.slack.com/archives/C1M8NH76X/p1596723935386100