Remove menu item pinning from Home and Favorites profile menus

Summary:
Ref T12174.

• Home now always uses the topmost item (falling back to "magic home") and no longer supports pinning. If any personal item may be a default item, it will always be picked over any global item.
• Favorites doesn't use defaults anyway, but no longer has misleading UI suggesting it might.

Test Plan:

• Saw no pinning UI on Home/Favorites.
• Added a personal dashboard on Home, it automatically became the new default.
• Pinned stuff normally on Projects.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12174

Differential Revision: https://secure.phabricator.com/D17298