Remove preference "Show table of contents (for pages with more ...)"

The table of contents box is auto-inserted, can trivially be hidden or
exposed on a per-page basis with __MAGICWORDS__, includes a sticky
[show|hide] link, and can be easily hidden with site-wide CSS as
necessary. It needlessly adds complexity and user interface clutter.

Bug: 52813
Change-Id: If2139317dae4aa980b373c73d7b81dac627b5af8
Merge "Remove preference "Show table of contents (for pages with more ...)""