Remove conversation window icon support

Removed the code related to gtk_window_set_icon_list. This code doesn't gain us much and will have even less impact in the future.

Testing Done:
Compiled, ran, verified that calling `gtk_window_set_icon_list(window, NULL);` doesn't crash. I couldn't actually check what a window manager will display because my window manager doesn't show icons.

Reviewed at https://reviews.imfreedom.org/r/77/