cmd/link: remove unnecessary use of sync.Once alongside sync.Mutex

There does not seem to be any point to this sync.Once.
I noticed because I was surveying uses of sync.Once to
understand usage patterns. This seems to be a dreg left over
from some earlier instance of the code.

Change-Id: I99dd258d865a41d0e8f6cfa55887855e477fb9c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/445755
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/link/internal/ld/errors.go

@@ -21,7 +21,6 @@ type symNameFn func(s loader.Sym) string
 // ErrorReporter is used to make error reporting thread safe.
 type ErrorReporter struct {
 	loader.ErrorReporter
-	unresOnce  sync.Once
 	unresSyms  map[unresolvedSymKey]bool
 	unresMutex sync.Mutex
 	SymName    symNameFn
@@ -29,11 +28,13 @@ type ErrorReporter struct {
 
 // errorUnresolved prints unresolved symbol error for rs that is referenced from s.
 func (reporter *ErrorReporter) errorUnresolved(ldr *loader.Loader, s, rs loader.Sym) {
-	reporter.unresOnce.Do(func() { reporter.unresSyms = make(map[unresolvedSymKey]bool) })
-
-	k := unresolvedSymKey{from: s, to: rs}
 	reporter.unresMutex.Lock()
 	defer reporter.unresMutex.Unlock()
+
+	if reporter.unresSyms == nil {
+		reporter.unresSyms = make(map[unresolvedSymKey]bool)
+	}
+	k := unresolvedSymKey{from: s, to: rs}
 	if !reporter.unresSyms[k] {
 		reporter.unresSyms[k] = true
 		name := ldr.SymName(rs)