If you do not want to change the default code then again in the style.css add at the end.
#branding img {
border: 0;
}
If ever you decide you want the ‘menu back’ it is easy just remove what you have added to the style.css, if you remove code you have to remember what you deleted.
As you are new to WordPress we should also point out that if the ‘twenty ten’ theme gets updated by WordPress and you press ‘yes’, like a few others did recently you could loose the changes you have made to your theme.
alchymyth has pointed out that removing the code saves loading time, however there is a strong opinion from WordPress that we should develop using child themes.
If you want to remove code then do have a read up on child themes, they are not hard once you understand the concept, and changes to the parent (twenty ten) will not affect the child theme if you update the parent.
HTH
Not everyone gets a quick reply, I just happened to be looking while at lunch.
David Child Themes Learn More