If you have copied the twentyeleven theme and are just customizing it its in the file content-page.php on line 13.
I personally have took out the whole block (starting with <header…) lines 12-14.
Alternatively if you are developing on Linux you can use grep/rgrep to search through source files in the command line and find whatever it is you are looking for, it can tell you what files and what lines it finds matches, that’s how I found it.
Please don’t just mute the title css its the lazy approach and sooner or later you will have to go back and do it properly. Save yourself the hassle later and do it properly from the start.
Happy coding
