org-table: several cleanups
* lisp/org.el (org-table-clean-did-remove-column),
lisp/org-table.el (org-table-clean-did-remove-column): Move defvar,
this dynamic variable is only used in org-table.
* lisp/org-table.el (org-table-colgroup-info): Remove unused defvar
for `org-table-colgroup-info'.
(org-table-clean-before-export): Let-bind regular expression strings
and remove unused matching group. Use
`org-table-clean-did-remove-column' in cond statement rather than
branching via if to avoid code duplication. Remove the code
associated with the removed `org-table-colgroup-info'.
(orgtbl-export): Remove unused internal function.