runtime: remove linkname for gopanic

github.com/goplus/igop now renamed github.com/goplus/ixgo
already requires checklinkname=0, so the special case can be removed.
https://github.com/goplus/ixgo/tree/e0d0bfeb2de9cfbe0b6cd668f015a1ba35dfea76/ixgo

go.undefinedlabs.com is no longer a resolvable domain,
having been absorbed by Datadog. The original use in
https://pkg.go.dev/go.undefinedlabs.com/scopeagent@v0.4.2/reflection
probably shouldn't have qualified anyway with 0 known importers.

For #67401

Change-Id: Ida6024e014f3304d4a4190f0bd9d12746a29b40b
Reviewed-on: https://go-review.googlesource.com/c/go/+/721300
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: seankhliao

src/runtime/panic.go

@@ -793,10 +793,7 @@ var panicnil = &godebugInc{name: "panicnil"}
 // The compiler emits calls to this function.
 //
 // gopanic should be an internal detail,
-// but widely used packages access it using linkname.
-// Notable members of the hall of shame include:
-//   - go.undefinedlabs.com/scopeagent
-//   - github.com/goplus/igop
+// but historically, widely used packages access it using linkname.
 //
 // Do not remove or change the type signature.
 // See go.dev/issue/67401.