The libpmem dependency that had been added in
commit 3daef523 (MDEV-17084)
did not achieve any measurable performance improvement when
comparing the same PMEM device with and without "mount -o dax"
using the Linux ext4 file system.

Because Red Hat has deprecated libpmem, let us remove the code
altogether.

Note: This is a 10.6 version of
commit 3f9f5ca4
which will retain PMEM support in MariaDB Server 10.11.