fix: hide right and left border on mobile dropdown (#4540)

hide right and left border on mobile
Co-authored-by: Charlie <charlie@uniswap.org>

src/components/NavBar/NavDropdown.css.ts

@@ -38,4 +38,8 @@ export const mobileNavDropdown = style([
     right: '0',
     width: 'full',
   }),
+  {
+    borderRightWidth: '0px',
+    borderLeftWidth: '0px',
+  },
 ])