removed the --dot line from the usage function

I removed a line related to an option that is not working on gegl anymore. The user in this issue: #216 asked why "--dot" is not working and if it was removed. After looking around in old commits, I found this one: f058da7e committed 9 years ago by Martin Nordholds, on which he says he removed the lines that check if "--dot" was input, however, he didn't remove the line from the usage function. Now, when one types gegl on command line, he will get an output that explains how to use it, however, the command "--dot" is still there, even though it does nothing. There is still a .c and .h files relate to what "--dot" should do, but as said by Martin above, it's not working as intended. Still, I didn't touch them because there is always the possibility of one fixing it. Given all that, I still think that the "--dot" line should be removed for now so that the user doesn't waste time trying to do something that do nothing. Maybe one should update the site to remove the line related to what the user in the issue link mentioned.