Stop "Header" fields (labels for form sections) from trying to generate Conduit edits

Summary: Fixes T12236. Headers are currently trying to generate an edit transaction for maniphest.edit and similar, but should not, since you can't edit them.

Test Plan:

• Configured Maniphest with a custom header field.
• Before change: maniphest.edit API console page fataled.
• After change: all good, no weird "header" transaction.
• Header still shows up on "Edit Task" form in web UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12236

Differential Revision: https://secure.phabricator.com/D17332