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

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

docs/code-quality/lint/linters/eslint-plugin-mozilla.rst

@@ -58,7 +58,6 @@ The plugin implements the following rules:
    eslint-plugin-mozilla/reject-lazy-imports-into-globals
    eslint-plugin-mozilla/reject-mixing-eager-and-lazy
    eslint-plugin-mozilla/reject-multiple-getters-calls
-   eslint-plugin-mozilla/reject-osfile
    eslint-plugin-mozilla/reject-relative-requires
    eslint-plugin-mozilla/reject-requires-await
    eslint-plugin-mozilla/reject-scriptableunicodeconverter

docs/code-quality/lint/linters/eslint-plugin-mozilla/reject-osfile.rst

@@ -1,13 +0,0 @@
-reject-osfile
-=============
-
-Rejects calls into ``OS.File`` and ``OS.Path``. This is configured as a warning.
-You should use |IOUtils|_ and |PathUtils|_ respectively for new code.  If
-modifying old code, please consider swapping it in if possible; if this is
-tricky please ensure a bug is on file.
-
-.. |IOUtils| replace:: ``IOUtils``
-.. _IOUtils: https://searchfox.org/mozilla-central/source/dom/chrome-webidl/IOUtils.webidl
-
-.. |PathUtils| replace:: ``PathUtils``
-.. _PathUtils: https://searchfox.org/mozilla-central/source/dom/chrome-webidl/PathUtils.webidl