Copy & Paste Following Code to Appearance > Customizer > Custom CSS

Case : Remove Border On Submenu

.nav .menu-item-has-children .sub-menu, .nav .menu-item .sub-menu .menu-item a {
border: none !important;
}

Case : Remove Border Botton On Link @ Submenu

.nav .menu-item .sub-menu .menu-item a, .nav .sub-menu .sub-menu .menu-item a {
border-bottom: none !important;
}

Case : Remove Padding On Submenu Link When Hovered

.nav .menu-item .sub-menu .menu-item:hover a {
padding: 0 !important;
}