gh-40710: Allow calling is_perfect() on immutable graphs
    
This is a simple fix to avoid an exception when calling `is_perfect()`
on an immutable graph.

The fix works by removing an unnecessary `remove_loops()` call added in
915a5e2.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have created tests covering the changes.
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preview.
    
URL: #40710
Reported by: Lennard Hofmann
Reviewer(s): David Coudert

Author: Release Manager

src/sage/graphs/graph.py

@@ -2670,7 +2670,7 @@ def is_perfect(self, certificate=False):
             sage: Graph(':Ab').is_perfect()                                             # needs sage.modules
             Traceback (most recent call last):
             ...
-            ValueError: This method is only defined for simple graphs, and yours is not one of them !
+            ValueError: This method is not known to work on graphs with multiedges/loops...
             sage: g = Graph()
             sage: g.allow_loops(True)
             sage: g.add_edge(0,0)
@@ -2679,18 +2679,21 @@ def is_perfect(self, certificate=False):
             sage: g.is_perfect()                                                        # needs sage.modules
             Traceback (most recent call last):
             ...
-            ValueError: This method is only defined for simple graphs, and yours is not one of them !
+            ValueError: This method is not known to work on graphs with loops. ...
+
+        TESTS:
+
+        Check that immutable graphs are supported::
+
+            sage: g = graphs.HouseGraph(immutable=True)
+            sage: g.is_perfect()                                                        # needs sage.modules
+            True
         """
-        if self.has_multiple_edges() or self.has_loops():
-            raise ValueError("This method is only defined for simple graphs,"
-                             " and yours is not one of them !")
+        self_complement = self.complement(immutable=False)
+
         if self.is_bipartite():
             return True if not certificate else None
 
-        self_complement = self.complement()
-        self_complement.remove_loops()
-        self_complement.remove_multiple_edges()
-
         if self_complement.is_bipartite():
             return True if not certificate else None