Maintainers.txt: Remove OvmfPkg/XenTimerDxe reference

Maintainers.txt: Remove OvmfPkg/XenTimerDxe reference

XenTimerDxe has been removed from the OvmfPkg.  Remove
file pattern for XenTimerDxe reviews from Maintainers.txt.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Andrew Fish <afish@apple.com>