qa: Cache only chain and wallet for regtest datadir

test/functional/test_framework/util.py

@@ -328,9 +328,6 @@ def get_auth_cookie(datadir):
         raise ValueError("No RPC credentials")
     return user, password
 
-def log_filename(dirname, n_node, logname):
-    return os.path.join(dirname, "node" + str(n_node), "regtest", logname)
-
 def get_bip9_status(node, key):
     info = node.getblockchaininfo()
     return info['bip9_softforks'][key]