Removed unused import.

tests/test_rules_poly.py

@@ -1,7 +1,6 @@
 from src.rules.poly import match_combine_polynomes, combine_polynomes
 from src.rules.numerics import add_numerics
 from src.possibilities import Possibility as P
-from src.node import ExpressionLeaf as L
 from src.parser import Parser
 from tests.parser import ParserWrapper
 from tests.rulestestcase import RulesTestCase