fix lint check (#746)

log/helper.go

@@ -4,8 +4,6 @@ import (
 	"fmt"
 )
 
-var nop Logger = new(nopLogger)
-
 // Helper is a logger helper.
 type Helper struct {
 	debug Logger

log/nop.go

@@ -1,5 +0,0 @@
-package log
-
-type nopLogger struct{}
-
-func (l *nopLogger) Print(kvpair ...interface{}) {}