docs: remove stale ValueError from expand() docstring

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: FBumann

flixopt/transform_accessor.py

@@ -1931,7 +1931,6 @@ def expand(self) -> FlowSystem:
 
         Raises:
             ValueError: If the FlowSystem was not created with ``cluster()``.
-            ValueError: If the FlowSystem has no solution.
 
         Examples:
             Two-stage optimization with expansion: