v1.6.1 to suppress warnings

### Release v1.6.1

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

Author: khoih-prog

CONTRIBUTING.md

@@ -15,7 +15,7 @@ If you don't find anything, please [open a new issue](https://github.com/khoih-p
 Please ensure to specify the following:
 
 * Arduino IDE version (e.g. 1.8.19) or Platform.io version
-* `Arduino megaAVR` or `MegaCoreX` Core Version (e.g. Arduino megaAVR core v1.8.7 or MegaCoreX core v1.0.9)
+* `Arduino megaAVR` or `MegaCoreX` Core Version (e.g. Arduino megaAVR core v1.8.7 or MegaCoreX core v1.0.10)
 * Contextual information (e.g. what you were trying to achieve)
 * Simplest possible steps to reproduce
 * Anything that might be relevant in your opinion, such as:
@@ -29,7 +29,7 @@ Please ensure to specify the following:
 Arduino IDE version: 1.8.19
 Arduino megaAVR Core Version 1.8.7
 OS: Ubuntu 20.04 LTS
-Linux xy-Inspiron-3593 5.4.0-97-generic #110-Ubuntu SMP Thu Jan 13 18:22:13 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
+Linux xy-Inspiron-3593 5.13.0-40-generic #45~20.04.1-Ubuntu SMP Mon Apr 4 09:38:31 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
 
 Context:
 I encountered a crash while trying to use the Timer Interrupt.