testing/internal/testdeps: delete didSetLogger variable

It was added in CL 83956 but never used.

Updates #23129

Change-Id: I70b50e974a56620069a77658386722af314cc857
Reviewed-on: https://go-review.googlesource.com/c/go/+/179138
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/testing/internal/testdeps/deps.go

@@ -98,7 +98,6 @@ func (l *testLog) add(op, name string) {
 }
 
 var log testLog
-var didSetLogger bool
 
 func (TestDeps) StartTestLog(w io.Writer) {
 	log.mu.Lock()