go.tools/go/ssa: remove obsolete comment.

This was a cryptic prediction of the bug you fixed. :)

LGTM=pcc
R=pcc
CC=golang-codereviews
https://golang.org/cl/103470048

go/ssa/builder.go

@@ -972,8 +972,6 @@ func (b *builder) assignStmt(fn *Function, lhss, rhss []ast.Expr, isDef bool) {
 		} else {
 			// Parallel assignment.  All reads must occur
 			// before all updates, precluding exprInPlace.
-			// TODO(adonovan): opt: is it sound to
-			// perform exprInPlace if !isDef?
 			var rvals []Value
 			for _, rval := range rhss {
 				rvals = append(rvals, b.expr(fn, rval))