Twenty Thirteen: Cleanup functions.php

[obenland]

​Based on Nacin's feedback, there are a few things that can be cleaned up:

• Missing period in sidebar description.
• Redundant conditional tags in twentythirteen_paging_nav().
• Further simplify twentythirteen_the_attached_image().
• Use add_editor_style() instead of adding a 'mce_css' callback (Requires a change to add_editor_style() where it accepts full URLs and escapes commas in those URLs).

Patch addresses:

• Adds missing period in sidebar description.
• Removes redundant conditional tags in twentythirteen_paging_nav().
• Further simplifies twentythirteen_the_attached_image().
• Removes braces from require statements (not a function).

In 24537:

Twenty Thirteen: cleanup for functions.php file, props obenland. See #24653.