test: speedup wallet_coinbase_category.py

No need to create a chain for it (nor use the cache).

test/functional/wallet_coinbase_category.py

@@ -15,6 +15,7 @@ from test_framework.util import (
 class CoinbaseCategoryTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
+        self.setup_clean_chain = True
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()