This fixes T97529: NLA track option buttons can be interacted with even when hidden.
The bug stems from the fact that there was duplicate input handling going on for the buttons: once in the normal button UI system, and then again in the mouse_nla_channels function. The logic in mouse_nla_channels does not inspect whether or not the setting button is there or not, it just assumes that it is.
This function should no longer be handling mouse input for buttons (there is even comment suggesting that the button handling to be deprecated) since the button UI system already handles it. Therefore, the button handling code has been removed from that mouse_nla_channels.
In addition, the redundant mouse button handling for pressing the "Push Down Action" button has also been removed from this function as well.