c++: Remove dg-ice in constexpr-52830.C [PR52830]

It turned out that r11-5942 fixed this old PR, and it was detected
by one of the few dg-ice tests -- exactly the point of them!  Now
the PR won't be opened until someone notices that it'd been fixed.
The patch failed to remove the dg-ice though, so now it XPASSes.  Fixing
this now.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-52830.C: Remove dg-ice.