OvmfPkg/License.txt: remove XenPvBlkDxe from the MIT licensed dir list

OvmfPkg/License.txt: remove XenPvBlkDxe from the MIT licensed dir list

All files under "OvmfPkg/XenPvBlkDxe" carry

  SPDX-License-Identifier: BSD-2-Clause-Patent

None of them are MIT-licensed.

Update "OvmfPkg/License.txt" accordingly.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: xen-devel@lists.xenproject.org
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1654
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>