memtx: remove `tree_iterator_set_current_tuple` on clarified tuples

All the 'base' TREE index iterator `next` methods (including
`tree_iterator_start`) internally set the iterator's current tuple to the
one found in the index satisfying the conditions: setting the iterator's
current tuple to the clarified one is redundant and moreover gives a
performance penalty each iteration because of the iterator check logic.

Needed for #7409

NO_CHANGELOG=refactoring
NO_DOC=refactoring
NO_TEST=refactoring