Updating data in HEAP table with BTREE index results in wrong index_length
counter value, which keeps growing after each update.

When inserting new record into tree counter is incremented by:
sizeof(TREE_ELEMENT) + key_size + tree->size_of_element
But when deleting element from tree it doesn't decrement counter by key_size:
sizeof(TREE_ELEMENT) + tree->size_of_element

This fix makes accurate allocated memory counter for tree. That is
decrease counter by key_size when deleting tree element.