Remove monospaced font preference global defaults

Summary: Fixes T7764. These settings have low utility, are no longer used by default, have become less useful on modern Windows which has a better selection of available fonts, and will eventually be subsumed (at least, for the most part) by T4103.

Test Plan:

• Grepped for strings.
• Viewed settings.
• Changed font to "24px impact".
• Viewed diffs with default and custom font.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: chad, epriestley

Maniphest Tasks: T7764

Differential Revision: https://secure.phabricator.com/D12301