Remove 'heading for processes' field on processes block

[helenb]

Link to Ticket

Description of Changes Made

Simply removes the field (which is not displayed anywhere in the front-end) and creates a migration to delete the content.
This is based off #549 to avoid any migration clashes, so that will need to be deployed first, or they can be deployed together.

How to Test

Edit a sub-proposition page, add a process block, and ensure you can't see a field called 'Heading for processes'.

Screenshots

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Before:

After:

MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• Tests and linting passes.
• Consider updating documentation. If you don't, tell us why.
• If relevant, list the environments / browsers in which you tested your changes.

[engineervix]

Looks good to me, thanks @helenb