The purpose of buf_pool->flush_rbt is to ensure that
buf_pool->flush_list is ordered by oldest_modification.
This should speed up multi-pass redo log application
(when the buffer pool is not large enough to accommodate
all pages that were modified since the latest log checkpoint).

The buf_pool->flush_rbt is not being used after redo log has
been applied. It could be better to always flush pages in
the ascending order of oldest_modification. Currently, whenever
a page is first modified, it will be moved to the start of the
buf_pool->flush_list, overtaking blocks whose oldest_modification
could be much older.

buf_flush_insert_sorted_into_flush_list(): Merge into
buf_flush_insert_into_flush_list().

buf_flush_recv_note_modification(): Remove.
The function buf_flush_note_modification() can be invoked instead.