(save-some-buffers): Simplify the fix for bug#46374

(save-some-buffers): Simplify the fix for bug#46374

* lisp/files.el (save-some-buffers): Only check the
`save-some-buffers-function` property on functions from
`save-some-buffers-default-predicate` since callers which provide
a `pred` argument can arrange to compute `pred` themselves if needed.

* test/lisp/files-tests.el (files-tests-buffer-offer-save): Don't test
with `pred` set to `save-some-buffers-root` since it's not an
appropriate function for that any more.