Revert "↑↓ for "Up" and "Down" (#5841)"

This reverts commit 566708b778.

Escape sequences are not legal key sequences.

Reference: https://doc.qt.io/qt-5/qkeysequence.html#details

src/playlist/queuemanager.ui

@@ -71,7 +71,7 @@
           </size>
          </property>
          <property name="shortcut">
-          <string>Ctrl+↑</string>
+          <string>Ctrl+Up</string>
          </property>
         </widget>
        </item>
@@ -90,7 +90,7 @@
           </size>
          </property>
          <property name="shortcut">
-          <string>Ctrl+↓</string>
+          <string>Ctrl+Down</string>
          </property>
         </widget>
        </item>