refactor: remove unused collapse icon from ObjectivesBoard

- Deleted the collapse icon SVG from the ObjectivesBoard component to clean up the code.
- This change simplifies the button layout and improves readability.

src/components/kanban/ObjectivesBoard.tsx

@@ -143,18 +143,6 @@ export function ObjectivesBoard({
                       {pinnedTagName}
                     </Badge>
                   )}
-                  
-                  {/* Flèche de collapse */}
-                  <svg
-                    className={`w-4 h-4 text-[var(--accent)] transition-transform duration-200 ${
-                      isCollapsed ? 'rotate-180' : ''
-                    }`}
-                    fill="none"
-                    stroke="currentColor"
-                    viewBox="0 0 24 24"
-                  >
-                    <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M19 9l-7 7-7-7" />
-                  </svg>
                 </button>
                 
                 <div className="flex items-center gap-2">