Remove deprecated warn kwarg for matplotlib use (#35323)

The warn param has been deprecated in 3.2.1: https://matplotlib.org/3.2.1/api/prev_api_changes/api_changes_3.2.0.html#matplotlib-use

Author: saulshanabrook

pandas/util/_test_decorators.py

@@ -94,7 +94,7 @@ def safe_import(mod_name: str, min_version: Optional[str] = None):
 def _skip_if_no_mpl():
     mod = safe_import("matplotlib")
     if mod:
-        mod.use("Agg", warn=True)
+        mod.use("Agg")
     else:
         return True