[dev.regabi] cmd/compile: remove Sym.Importdef

Evidently it hasn't been needed since circa 2018, when we removed the
binary export data format.

Change-Id: I4e4c788d6b6233340fb0de0a56d035c31d96f761
Reviewed-on: https://go-review.googlesource.com/c/go/+/280634


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>