libgomp.texi: Remove now duplicate TR13 item

Remove an item under "Other new TR 13 features" that since the last commit
(r15-3917-g6b7eaec20b046e) to this file is is covered by the added
  "New @code{storage} map-type modifier; context-dependent @code{alloc} and
   @code{release} are aliases"
  "Update of the map-type decay for mapping and @code{declare_mapper}"

libgomp/
	* libgomp.texi (TR13 status): Update semi-duplicated, semi-obsoleted
	item; remove left-over half-sentence.