Remove `export-block' syntax type

Remove `export-block' syntax type

* lisp/org-element.el (org-element-all-elements): Remove
  `export-block' type.
(org-element-block-name-alist): Make it a defconst since it is not
meant to be changed anymore.
(org-element-export-block-parser,
org-element-export-block-interpreter): Remove functions.

* lisp/ox.el (org-export-filters-alist): Remove export block filter.
(org-export-register-backend): Do not alter
`org-export-block-name-alist' anymore.
(org-export-define-backend, org-export-define-derived-backend): Update
docstring.
(org-export-filter-export-block-functions): Remove variable.

* testing/lisp/test-org-element.el (test-org-element/export-block-parser,
  test-org-element/export-block-interpreter): Remove tests.

* testing/lisp/test-ox.el (test-org-export/define-backend): Update test.