This patch removes the vast majority of OS.File and support code. A few things remain:

- The nsIOSFileConstantsService still exists, but the path related constants
  (OS.Constants.Path.*) are no longer added to the OS object. The plan is to
  replace this with a proper service e.g. Services.osConstants or similar) in
  bug 1786885.
- There is still support for OS.File errors in ErrorSanitizer, which will be
  removed in bug 1775167.
- The OS.File to IOUtils migration guide will be rewritten as general IOUtils
  documentation in bug 1830097.
- dom/base/Document.cpp has a workaround for not loading osfile.jsm at startup,
  which may want to be reconsidered in bug 1830100.

So long, and thanks for all the I/O.

Differential Revision: https://phabricator.services.mozilla.com/D176543