Let eglot-flymake-backend be in flymake-d-functions even if eglot off

Let eglot-flymake-backend be in flymake-d-functions even if eglot off

This is useful when using  eglot-stay-out-of and a pattern like:

   (defun my/js-mode-hook ()
     (add-hook 'flymake-diagnostic-functions 'some-eslint-backend nil t))
     (setq-local eglot-stay-out-of '(flymake))
     (add-hook 'flymake-diagnostic-functions 'eglot-flymake-backend nil t))

   (add-hook 'js-mode-hook 'my/js-mode-hook)

Then, _both_ backends will run unconditionally, but Eglot backend only
actually reports diagnostics if Eglot is on.

* eglot.el (eglot-flymake-backend): If buffer isn't being managed
by Eglot, behave as a noop.