[mac] remove unused method definitions (#3374)
[mac] remove unused method definitions (#3374)

This class removes unused method definitions in `Mac` class. Note
that these methods are now provided and used in `SubMac`.
srickardti/openthread,chshu/openthread,chshu/openthread,bukepo/openthread,librasungirl/openthread,chshu/openthread,openthread/openthread,openthread/openthread,jwhui/openthread,erja-gp/openthread,abtink/openthread,chshu/openthread,erja-gp/openthread,erja-gp/openthread,erja-gp/openthread,turon/openthread,chshu/openthread,turon/openthread,abtink/openthread,lanyuwen/openthread,lanyuwen/openthread,turon/openthread,bukepo/openthread,librasungirl/openthread,turon/openthread,chshu/openthread,srickardti/openthread,librasungirl/openthread,librasungirl/openthread,bukepo/openthread,lanyuwen/openthread,srickardti/openthread,erja-gp/openthread,lanyuwen/openthread,erja-gp/openthread,openthread/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread,abtink/openthread,lanyuwen/openthread,openthread/openthread,bukepo/openthread