After taking a look at Drupal 7, it seems as though this line you are referring to is added by the following:
.pager a, .view-latest-from-blog li, .comment, .tabs ul.primary,
.tabs ul.primary li a, .tabs ul.primary li.active a, .tabs ul.primary li a:hover,
.meta-share-wrap, .meta_share, #footer {
border-color: #e4e4e4;
}
In your case, it's being applied to the #footer
element, therefore you would either remove the #footer
selector from the initial declaration, or you would use the following to overwrite it:
#main-content #footer {
border: none;
}