docs/GithubMove.rst: Remove obsolete information

Summary:
Remove references to the multirepo and update the document to
reflect the current state of the github repository.

Reviewers: mehdi_amini, jyknight

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58420

llvm-svn: 365645