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/ipc.c

@@ -268,10 +268,6 @@ void dump_node(yajl_gen gen, struct Con *con, bool inplace_restart) {
         case CT_DOCKAREA:
             ystr("dockarea");
             break;
-        default:
-            DLOG("About to dump unknown container type=%d. This is a bug.\n", con->type);
-            assert(false);
-            break;
     }
 
     /* provided for backwards compatibility only. */