Remove `org-list-empty-line-terminates-plain-lists'
* lisp/org-list.el (org-list-empty-line-terminates-plain-lists): Remove
variable.
(org-list-end-re):
(org-list-separating-blank-lines-number):
* lisp/org.el (org-blank-before-new-entry): Adapt to change above.
* testing/lisp/test-org-element.el (test-org-element/at-point):
* testing/lisp/test-org-list.el (test-org-list/move-item-down):
(test-org-list/move-item-up):
(test-org-list/insert-item): Update tests.
This variable is not useful: plain lists can always be terminated with
two blank lines.