Prefer compiler warnings to assertions for unhandled switch cases

Using 'default:' cases can hide logical errors which would lead to i3
crashes for users. With this change the compiler will print a warning
when a case is not handled. For example, if I add a new value in the
Font.type enum:
../../i3/libi3/font.c: In function ‘draw_text’:
../../i3/libi3/font.c:378:5: warning: enumeration value ‘NEWFONT’ not handled in switch [-Wswitch]
     switch (savedFont->type) {
     ^~~~~~

src/click.c

@@ -44,9 +44,6 @@ static bool tiling_resize_for_border(Con *con, border_t border, xcb_button_press
         case BORDER_BOTTOM:
             search_direction = D_DOWN;
             break;
-        default:
-            assert(false);
-            break;
     }
 
     bool res = resize_find_tiling_participants(&first, &second, search_direction, false);