Revert "x11: get x1/x2 button state in GetGlobalMouseState"

This reverts commit 3fcc2cb500a752698687ad0c8bb14778e6cb84c3.

Button4 and Button5 are for the scrollwheel, not the extended buttons.
I don't know of a way to query the state of the extended buttons using X11.

src/video/x11/SDL_x11mouse.c

@@ -406,8 +406,6 @@ X11_GetGlobalMouseState(int *x, int *y)
                     buttons |= (mask & Button1Mask) ? SDL_BUTTON_LMASK : 0;
                     buttons |= (mask & Button2Mask) ? SDL_BUTTON_MMASK : 0;
                     buttons |= (mask & Button3Mask) ? SDL_BUTTON_RMASK : 0;
-                    buttons |= (mask & Button4Mask) ? SDL_BUTTON_X1MASK : 0;
-                    buttons |= (mask & Button5Mask) ? SDL_BUTTON_X2MASK : 0;
                     /* SDL_DisplayData->x,y point to screen origin, and adding them to mouse coordinates relative to root window doesn't do the right thing
                      * (observed on dual monitor setup with primary display being the rightmost one - mouse was offset to the right).
                      *