Remove unnecessary story-points field

For some reason, despite using Maniphest's built-in story-points field,
the extension continued to provide its own, which resulted in duplicate
fields showing in the UI.

Remove it, and all remenants of code using it, to avoid confusion.

Signed-off-by: Daniel Stone <daniels@collabora.com>

Differential Revision: https://phabricator.collabora.co.uk/D483