internal/vulncheck: remove unnecessary assignment to the blank identi…

…fier

This leads to slightly simpler code, and
fixes failing TestStaticCheck as reported by longtest builders.

Change-Id: I2631dec5ef657defcf9052cb5a3b48cc68feccd9
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/505597
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

internal/vulncheck/witness.go

@@ -288,7 +288,7 @@ func funcLess(f1, f2 *FuncNode) bool {
 }
 
 func filterCallStacks(callstacks map[*Vuln][]CallStack) {
-	for vv, _ := range callstacks {
+	for vv := range callstacks {
 		if vv.CallSink != nil {
 			if len(callstacks[vv]) > 0 {
 				callstacks[vv] = callstacks[vv][0:1]