v1.6.1 to suppress warnings

### Release v1.6.1

1. Suppress warnings when `_TIMERINTERRUPT_LOGLEVEL_` < 2
2. Remove unused `_prescalerIndex`

Author: khoih-prog

README.md

@@ -2,7 +2,7 @@
 
 [![arduino-library-badge](https://www.ardu-badge.com/badge/megaAVR_TimerInterrupt.svg?)](https://www.ardu-badge.com/megaAVR_TimerInterrupt)
 [![GitHub release](https://img.shields.io/github/release/khoih-prog/megaAVR_TimerInterrupt.svg)](https://github.com/khoih-prog/megaAVR_TimerInterrupt/releases)
-[![GitHub](https://img.shields.io/github/license/mashape/apistatus.svg)](https://github.com/khoih-prog/megaAVR_TimerInterrupt/blob/master/LICENSE)
+[![GitHub](https://img.shields.io/github/license/mashape/apistatus.svg)](https://github.com/khoih-prog/megaAVR_TimerInterrupt/blob/main/LICENSE)
 [![contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)](#Contributing)
 [![GitHub issues](https://img.shields.io/github/issues/khoih-prog/megaAVR_TimerInterrupt.svg)](http://github.com/khoih-prog/megaAVR_TimerInterrupt/issues)
 
@@ -1150,7 +1150,7 @@ If you want to contribute to this project:
 
 ## License
 
-- The library is licensed under [MIT](https://github.com/khoih-prog/megaAVR_TimerInterrupt/blob/master/LICENSE)
+- The library is licensed under [MIT](https://github.com/khoih-prog/megaAVR_TimerInterrupt/blob/main/LICENSE)
 
 ---