Remove vestiges of support for stdin input to abcfield.py.

I'm not using it, and it complicates things a bit.

abcfield.py

@@ -285,11 +285,7 @@ if __name__ == "__main__":
                         help='input ABC file')
     args = parser.parse_args()
 
-    res = False
-    if args.input:
     path = pathlib.Path(args.input.name)
     with path.open() as f:
         res = process(f, path.parent, args)
-    else:
-        res = process(sys.stdin, ".", args)
     sys.exit(int(not res))