remove unhelpful assertion (IDEA-73526)

platform/platform-api/src/com/intellij/openapi/actionSystem/MouseShortcut.java

@@ -28,13 +28,6 @@ public final class MouseShortcut extends Shortcut {
   private final int myClickCount;
 
   public MouseShortcut(int button, int modifiers, int clickCount) {
-    if (
-      MouseEvent.BUTTON1 != button &&
-      MouseEvent.BUTTON2 != button &&
-      MouseEvent.BUTTON3 != button
-    ) {
-      throw new IllegalArgumentException("unknown button: " + button);
-    }
     myButton = button;
     // TODO[vova] check modifiers?
     myModifiers = mapOldModifiers(modifiers);

platform/platform-api/src/com/intellij/openapi/keymap/KeymapUtil.java

@@ -128,7 +128,7 @@ public class KeymapUtil {
       // do nothing
     }
     else {
-      throw new IllegalStateException("unknown button: " + button);
+      buttonNum = button;
     }
 
     if (clickCount == 1) {

platform/platform-impl/src/com/intellij/openapi/keymap/impl/KeymapImpl.java

@@ -801,7 +801,7 @@ public class KeymapImpl implements Keymap, ExternalizableScheme {
       buffer.append(' ');
     }
     else {
-      throw new IllegalStateException("unknown button: " + button);
+      buffer.append("button").append(button);
     }
 
     if (shortcut.getClickCount() > 1) {