org-element: code clean-up and some optimizations

org-element: code clean-up and some optimizations

* contrib/lisp/org-element.el (org-element-center-block-parser,
  org-element-plain-list-parser, org-element-quote-block-parser,
  org-element-special-block-parser, org-element-comment-parser,
  org-element-comment-block-parser, org-element-example-block-parser,
  org-element-export-block-parser, org-element-fixed-width-parser,
  org-element-horizontal-rule-parser, org-element-keyword-parser,
  org-element-latex-environment-parser, org-element-paragraph-parser,
  org-element-property-drawer-parser, org-element-src-block-parser,
  org-element-table-parser): Since parser will alway be called at the
  beginning of the element, simplify code.