Do not send message to resize iFrame based on cell actions

This partially reverts: https://github.com/twosigma/beaker-notebook/commit/ec1d377fe990efea465853727b7ab7802375d80e
Send message to resize iFrame when notebook's actual height changes
Merge pull request #888 from twosigma/bg/iframe_resize

[bunsen] Send message to resize iFrame when notebook's actual height changes