buf_dblwr_process(): Remove the useless warning that a copy of a page
in the doublewrite buffer is corrupted. We already report an error if a
corrupted page cannot be recovered from the doublewrite buffer.

Note: In MariaDB 10.1, the original bug reported in MDEV-13893 could
still be easily repeatable. In MariaDB 10.2.24, MDEV-12699 should
have reduced the probability considerably.