remove dead code.

buf_flush_ready_for_flush(), buf_flush_try_page(): Assert that
flush_type is one of BUF_FLUSH_LRU or BUF_FLUSH_LIST.  The flush_type
comes from buf_flush_batch(), which already asserts this.  The
assertion holds for all calls in the source code.

buf_flush_try_page(): Remove the dead case BUF_FLUSH_SINGLE_PAGE
of switch (flush_type).