Fix HentaiVN

Remove some warning

src/main/kotlin/org/koitharu/kotatsu/parsers/site/vi/HentaiVNParser.kt

@@ -5,14 +5,12 @@ import kotlinx.coroutines.async
 import kotlinx.coroutines.coroutineScope
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
-import okhttp3.Headers
 import org.jsoup.nodes.Document
 import org.koitharu.kotatsu.parsers.MangaLoaderContext
 import org.koitharu.kotatsu.parsers.MangaParser
 import org.koitharu.kotatsu.parsers.MangaSourceParser
 import org.koitharu.kotatsu.parsers.config.ConfigKey
 import org.koitharu.kotatsu.parsers.model.*
-import org.koitharu.kotatsu.parsers.network.UserAgents
 import org.koitharu.kotatsu.parsers.util.*
 import java.text.SimpleDateFormat
 import java.util.*