ImageJ: remove redundant input() method

It is part of the AbstractGateway now.
WIP: Make AWT and Swing projects external

See:
* https://github.com/imagej/imagej-ui-awt
* https://github.com/imagej/imagej-ui-swing
* https://github.com/scijava/scijava-ui-awt
* https://github.com/scijava/scijava-ui-swing