chore: fix linter warnings

cmd/sslcert/main.go

@@ -159,7 +159,6 @@ type programFlags struct {
 type checkProgram struct {
 	programFlags                   // Flags from the command line
 	plugin       *plugin.Plugin    // Plugin output state
-	getter       certGetter        // Certificate getter
 	certificate  *x509.Certificate // X.509 certificate from the server
 }
 
@@ -338,7 +337,7 @@ func (program *checkProgram) RunCheck() {
 	if err != nil {
 		program.plugin.SetState(plugin.UNKNOWN, err.Error())
 	} else if program.checkNames() {
-		timeLeft := program.certificate.NotAfter.Sub(time.Now())
+		timeLeft := time.Until(program.certificate.NotAfter)
 		tlDays := int((timeLeft + 86399*time.Second) / (24 * time.Hour))
 		program.plugin.SetState(program.checkCertificateExpiry(tlDays))
 		program.setPerfData(tlDays)