Remove additional listener in export-and-sign

[leeland-turnkey]

We have an additional listener that is a duplicate of code that is already running in event-handler.js file. This code in index.js file is a duplicate of what is running in this code

TURNKEY_INIT_MESSAGE_CHANNEL is handled by initEventHandlers() which is called in DOMContentLoaded above.
The initEventHandlers function sets up a message listener for TURNKEY_INIT_MESSAGE_CHANNEL internally.

Here is a color representation of the logs before and after the delete.

Before:
Color map of console.log

• Green: https://github.com/tkhq/frames/blob/main/export-and-sign/src/index.js#L13
• Red: https://github.com/tkhq/frames/blob/main/export-and-sign/src/index.js#L32
• Orange: https://github.com/tkhq/frames/blob/main/export-and-sign/src/event-handlers.js#L611

After:
Color map of console.log

• Red: https://github.com/tkhq/frames/blob/main/export-and-sign/src/index.js#L13
• Orange: https://github.com/tkhq/frames/blob/main/export-and-sign/src/event-handlers.js#L611

[ethankonk]

Some merge conflicts you'll have to fix but looks great so far!

[leeland-turnkey]

@ethankonk Pulled in main so it should be good to go!