(loadhist-unload-element): Move ERT and cl-generic methods

(loadhist-unload-element): Move ERT and cl-generic methods

* lisp/loadhist.el (loadhist-unload-element): Don't define cl-generic
and ert methods here.
(loadhist-unload-element) <(head define-type)>: Remove unused var `slots'.

* lisp/emacs-lisp/cl-generic.el (loadhist-unload-element): Define
unload method for cl-defmethod.
(cl-generic-ensure-function): Remove redundant `defalias'.

* lisp/emacs-lisp/ert.el (ert-set-test): Move the current-load-list
setting here...
(ert-deftest): ...from here.
(loadhist-unload-element): Define unload method for ert-deftest.