runtime: remove comment about gcCopySpans()

Remove documentation reference to gcCopySpans(), as that function was
removed in https://golang.org/cl/30537

Fixes #35683

Change-Id: I7fb7c6cc60bfb3a133a019a20eb3f9d4c7627b31
Reviewed-on: https://go-review.googlesource.com/c/go/+/209917Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgcmark.go

@@ -46,8 +46,6 @@ const (
 // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
 // some miscellany) and initializes scanning-related state.
 //
-// The caller must have call gcCopySpans().
-//
 // The world must be stopped.
 //
 //go:nowritebarrier