Bug 1776480 - Remove OS.File et al. r=Gijs,webidl,smaug

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

browser/base/content/test/performance/browser_startup.js

@@ -102,7 +102,6 @@ const startupPhases = {
       modules: new Set([
         "resource://gre/modules/AsyncPrefs.sys.mjs",
         "resource://gre/modules/LoginManagerContextMenu.sys.mjs",
-        "resource://gre/modules/osfile.jsm",
         "resource://pdf.js/PdfStreamConverter.sys.mjs",
       ]),
     },

browser/base/content/test/static/browser_all_files_referenced.js

@@ -71,12 +71,6 @@ var gExceptionPaths = [
 
   // Localization file added programatically in featureCallout.jsm
   "resource://app/localization/en-US/browser/featureCallout.ftl",
-
-  // Will be removed in bug 1737308
-  "resource://gre/modules/lz4.js",
-  "resource://gre/modules/lz4_internal.js",
-  "resource://gre/modules/osfile.jsm",
-  "resource://gre/modules/osfile/",
 ];
 
 // These are not part of the omni.ja file, so we find them only when running