Remove the old, non-modular Excel export workflow from Maniphest

Summary:
Depends on D18960. Ref T13049. Now that Maniphest fully supports "Export Data", remove the old hard-coded version.

This is a backward compatibility break with the handful of installs that might have defined a custom export by subclassing ManiphestExcelFormat. I suspect this is almost zero installs, and that the additional data in the new format may serve most of the needs of this tiny number of installs. They can upgrade to ExportEngineExtensions fairly easily if this isn't true.

Test Plan:

• Viewed Maniphest, no longer saw the old export workflow.
• Grepped for export and similar strings to try to hunt everything down.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18961