Remove duplicate ATtiny1634 definition
Created by: fauxpark
The ATtiny1634-MN and -MU are, for the purposes of the libraries, completely identical. The N/U suffix indicates the lead finish on the package itself. U (matte tin) is the most common here so I have chosen that one as the one to keep. Obviously they should be removed from all of the symbol names, but of course that kind of mass renaming must wait until later in the development cycle.