MDEV-20813: Remove the buf_flush_init_for_writing() assertion

Old InnoDB/XtraDB versions only initialized FIL_PAGE_TYPE for
B-tree pages (to FIL_PAGE_INDEX), and left it uninitialized
(possibly containing FIL_PAGE_INDEX) for others. In MySQL
or MariaDB 5.5, the field is initialized on almost all pages,
but still not all of them.

In MariaDB 10.2 and later, buf_flush_init_for_writing() would
initialize the FIL_PAGE_TYPE on such old pages, but only after
passing the debug assertion that we are now removing from 10.1.
There, we will be able to modify fil_crypt_rotate_page() so
that it will skip the key rotation for pages that contain 0
in FIL_PAGE_TYPE.

In MariaDB 10.1, there is no logic that would initialize
FIL_PAGE_TYPE on data pages in old data files after an update.
So, encryption key rotation may routinely cause page flushes
on pages that contain 0 in FIL_PAGE_TYPE.

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