Don't try to center dialogs horizontally

Summary:
Fixes T10302. I think we had fixed-width dialog containers in the past (?) but they all handle their own centering now.

This was causing them to be slightly off-center as a result, and creating the 7px issue in T10302.

Test Plan:

• Viewed a wide dialog (task edit).
• Viewed a narrow dialog (notification dismissal confirmation).
• Viewed dialogs on wide/narrow screens.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10302

Differential Revision: https://secure.phabricator.com/D15529