compile warning: ElaborationArtefactAnnotation remove unreachable code

[ingallsj]

Related issue: introduced by #2729

Type of change: paying off technical debt

Impact: no functional change

Development Phase: implementation

Release Notes
fix compile warning:

[warn] /rocket-chip/src/main/scala/util/ElaborationArtefactAnnotation.scala:80:27: unreachable code
[warn]       case Some(other) => throw new Exception(s"memory $target cannot be renamed to $other")
[warn]                           ^