Do not set output->current_mode in compositor.c

The field is already set - correctly - in the backend switch_mode.
setting output->current_mode to mode in compositor.c leads to bugs,
since mode can be freed by the shell.
For example, the shell allocates it on the stack for
WL_SHELL_SURFACE_FULLSCREEN_METHOD_DRIVER

Signed-off-by: Axel Davy <axel.davy@ens.fr>

src/compositor.c

@@ -137,7 +137,6 @@ weston_output_switch_mode(struct weston_output *output, struct weston_mode *mode
 		if (ret < 0)
 			return ret;
 
-		output->current_mode = mode;
 		output->current_scale = scale;
 		break;
 	case WESTON_MODE_SWITCH_RESTORE_NATIVE: