fortran: Remove redundant tree walk to delete element

fortran: Remove redundant tree walk to delete element

Remove preliminary walk of the symbol tree when we are about to remove an
element.  This preliminary walk was necessary because the deletion function
updated the tree without reporting back to the caller the element it had
removed.  But knowing that element is necessary to free its memory, so one
had to first get that element before it was removed from the tree.

This change updates the main deletion function delete_treap and its public
wrapper gfc_delete_bbt so that the removed element can be known by the
caller.  This makes the preliminary walk in gfc_delete_symtree redundant,
permitting its removal.

gcc/fortran/ChangeLog:

	* bbt.cc (delete_treap): Add argument REMOVED, set it to the removed
	element from the tree.  Change NULL to nullptr.
	(gfc_delete_bbt): Return the removed element from the tree.
	* gfortran.h (gfc_delete_symtree): Remove prototype.
	(gfc_delete_bbt): Set return type to pointer.
	* symbol.cc (gfc_delete_symtree): Make static.  Get the element to be
	freed from the result of gfc_delete_bbt.  Remove the preliminary walk to
	get it.