[dev.regabi] cmd/compile: remove race-y check in Name.Canonical

The backend doesn't synchronize compilation of functions with their
enclosed function literals, so it's not safe to double-check that
IsClosureVar isn't set on the underlying Name. Plenty of frontend
stuff would blow-up if this was wrong anyway, so it should be fine to
omit.

Change-Id: I3e97b64051fe56d97bf316c9b5dcce61f2082428
Reviewed-on: https://go-review.googlesource.com/c/go/+/281812


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>