[python] remove useless docstring for models (#6138)

fix #6115

in our current design, read-only are not exposed as __init__ kwarg like
it used to be (e.g.
[here](https://github.com/Azure/autorest.python/blob/ac9d2bcd5b7965258a5d4094c78fd234ab31f030/packages/typespec-python/test/azure/generated/headasbooleantrue/headasbooleantrue/models/_models.py#L33-L70)).
So we could remove the sentence `Readonly variables are only populated
by the server, and will be ignored when sending a request` now.

.chronus/changes/python-models-docstring-2025-1-25-2-51-40.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+[python] remove useless docstring for models

packages/http-client-python/generator/pygen/codegen/templates/model_dpg.py.jinja2

@@ -8,10 +8,6 @@
 
     {{ serializer.discriminator_docstring(model) | wordwrap(width=95, break_long_words=False, break_on_hyphens=False, wrapstring='\n    ') }}
     {% endif %}
-    {% if model.has_readonly_or_constant_property %}
-
-    Readonly variables are only populated by the server, and will be ignored when sending a request.
-    {% endif %}
 
     {% if model.properties != None %}
         {% for p in model.properties %}