Remove misleading duplicated error message

The correct error message has been printed by getopt_long()
before returning.

Test cases:
$ rigmem -Q
rigmem: invalid option -- 'Q'
Unknown option '?'
Usage: rigmem [OPTION]... COMMAND... FILE
...

$ rigmem --foo
rigmem: unrecognized option '--foo'
Unknown option '?'
Usage: rigmem [OPTION]... COMMAND... FILE
...

tests/rigmem.c

@@ -187,7 +187,6 @@ int main(int argc, char *argv[])
             break;
 
         default:
-            fprintf(stderr, "Unknown option '%c'\n", c);
             usage();    /* unknown option? */
             exit(1);
         }