To hide the UI for the embedded prototype, you need to edit the URL line by adding the &hide-ui=1 parameter.
For example:
<iframe src="https://www.figma.com/embed?embed_host=share&url=https%3A%2F%2Fwww.figma.com%2Fproto%2FFileKey%2FFileName%3Fnode-id%3D1%253A2%26scaling%3Dscale-down%26page-id%3D0%253A1%26hide-ui%3D1"></iframe>