Remove legacy "DashboardInstall" table

Summary: Depends on D20409. Ref T13272. Before "ProfileMenu", dashboards were installed on specific objects using this table. Installs are now handled via ProfileMenu and this table no longer has any meaningful readers. Remove references to the table and destroy it.

Test Plan: Grepped for DashboardInstall.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20410