(stable) When a user cancels a new inline, clear it from the objective list

Summary: Ref T12733. Currently, creating a new inline and then canceling it leaves a marker in the objective list. Instead, remove the marker.

Test Plan:

• Created an empty inline, cancelled. Created a non-empty inline, cancelled. No objective marker in either case.
• Created a new normal inline, objective marker.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D17979