Remove ‘ert-with-function-mocked’ macro in favour of ‘cl-letf’ macro

Remove ‘ert-with-function-mocked’ macro in favour of ‘cl-letf’ macro

* lisp/emacs-lisp/ert-x.el (ert-with-function-mocked): Remove macro
in favour of ‘cl-letf’ macro which is more generic.  All existing
uses are migrated accordingly.  The macro has not been included in
an official release yet so it should be fine to delete it.