Remove alt-mousewheel bindings for volume adjustment

With the recent changes to the order of processing key bindings
(`GlobalAction`s are handled first), having the alt-wheel bindings in here causes a
regression as they are handled before `OnScroll` events.

Specifically, this means editor alt-scroll functionality no longer works
with the default bindings.

Removing the bindings fixes this, while also still allowing alt-wheel
adjustment of the volume via `VolumeControlReceptor`:

a2f50af424/osu.Game/Overlays/Volume/VolumeControlReceptor.cs (L21-L26)

In conjunction with the special case in `OsuScrollContainer`:

02d5b1352b/osu.Game/Graphics/Containers/OsuScrollContainer.cs (L103-L105)

osu.Game/Migrations/20210412045700_RefreshVolumeBindingsAgain.cs

@@ -0,0 +1,16 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+namespace osu.Game.Migrations
+{
+    public partial class RefreshVolumeBindingsAgain : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql("DELETE FROM KeyBinding WHERE action in (6,7)");
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+        }
+    }
+}