The test case for reproducing MDEV-14126 demonstrates that InnoDB can
end up with an index tree where a non-leaf page has only one child page.

The test case innodb.innodb_bug14676111 demonstrates that such pages
are sometimes unavoidable, because InnoDB does not implement any sort
of B-tree rotation.

But, there is no reason to allow a root page with only one child page.

btr_cur_node_ptr_delete(): Replaces btr_node_ptr_delete().

btr_page_get_father(): Declare globally.

btr_discard_only_page_on_level(): Declare with ATTRIBUTE_COLD.
It turns out that this function is not covered by the
innodb.innodb_bug14676111 test case after all.

btr_discard_page(): If the root page ends up having only one child
page, shrink the tree by invoking btr_lift_page_up().