docs: Removed misleading sync adapter section.

Removed section "Run the Sync Adapter After a Network Message," as
several members of DevRel believe that the explanation and code
sample were too misleading. Also removed related TOC entry and
"dictionary entry" (within the user-friendly list of options for sync
adapters) at the end of the introductory section for this topic.

Bug: 19718330
Change-Id: I1cd2c2e5253653f670c62be95ab50ab626d26a56