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.

storage/innobase/buf/buf0flu.cc

@@ -745,7 +745,6 @@ buf_flush_init_for_writing(
 	ib_uint32_t	checksum = 0 /* silence bogus gcc warning */;
 
 	ut_ad(page);
-	ut_ad(!newest_lsn || fil_page_get_type(page));
 
 	if (page_zip_) {
 		page_zip_des_t*	page_zip;

storage/xtradb/buf/buf0flu.cc

@@ -787,7 +787,6 @@ buf_flush_init_for_writing(
 	ib_uint32_t	checksum = 0 /* silence bogus gcc warning */;
 
 	ut_ad(page);
-	ut_ad(!newest_lsn || fil_page_get_type(page));
 
 	if (page_zip_) {
 		page_zip_des_t*	page_zip;