[region-isolation] Remove Partition::getRegion().

This was a piece of code that I added early while adding better unittesting for
Partition. Instead in that patch I added a forward declared class in Partition
called PartitionTester that could implement getRegion. I just forgot to remove
this.

Author: gottesmm

include/swift/SILOptimizer/Utils/PartitionUtils.h

@@ -634,9 +634,6 @@ class Partition {
     }
     os << "]\n";
   }
-
-  /// Routine used in unittests
-  Region getRegion(Element elt) const { return labels.at(elt); }
 };
 
 } // namespace swift