OvmfPkg/OvmfXen: Remove SEC source level debug for Xen

Xen uses PcdFSBClock as a dynamic pcd, but it cannot be accessed in
SEC phase. This commit disables source level debug for Xen's SEC.
For Xen's SEC debugging, use normal OVMF instead.

Signed-off-by: maouai233 <maouai233@outlook.com>