[refactor] Turn off stack traceback info by default (#5347)

When Taichi program crashes we used to show a `Taichi Compiler Stack
Traceback` (a.k.a purple stack) but this stack doesn't have enough symbol for developers, nor
it's readable to normal Taichi python users. So let's turn it off by
default to avoid confusing users.

Just in case someone still needs it, it can be brought back if you do
`ti.init(debug=True)`.

Author: ailzhang

python/taichi/lang/misc.py

@@ -454,7 +454,7 @@ def init(arch=None,
 
     impl._root_fb = _snode.FieldsBuilder()
 
-    if not os.environ.get("TI_DISABLE_SIGNAL_HANDLERS", False):
+    if cfg.debug:
         impl.get_runtime()._register_signal_handlers()
 
     # Recover the current working directory (https://github.com/taichi-dev/taichi/issues/4811)