Remove Data.setRef assertion (bp #1654) (#1656)

* Remove Data.setRef assertion (#1654)

It causes issues for some legal (if awkward) patterns. A larger refactor
of when refs are set could reinstate this check.

(cherry picked from commit 679dd54a

)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Fix merge conflict

Co-authored-by: Jack Koenig <koenig@sifive.com>