Convconst is not used in the new backend, and all its callers
were deleted in CL 29168 (cmd/compile: delete lots of the legacy
backend). iconv was an helper function for Convconst.

Updates #16357

Change-Id: I65c7345586d7af81cdc2fb09c68f744ffb161a17
Reviewed-on: https://go-review.googlesource.com/30090Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>