[1.6] Remove table of contents at the top of rpc.rst (#40482)
Master PR: https://github.com/pytorch/pytorch/pull/40205
Remove the table of contents created by the `.. contents:: :local: :depth: 2` since this page isn't one of the large documentation pages (https://github.com/pytorch/pytorch/issues/38010) and is simply a landing page for the Distributed RPC Framework.
Changes made in this original PR: https://github.com/pytorch/pytorch/commit/f10fbcc820d1507121ed466f3dffc728ef559c5c#diff-250b9b23fd6f1a5c15aecdb72afb9d7d
