test: remove -spendzeroconfchange setting from mempool_limit.py

Since this test was changed to use MiniWallet instead of the Bitcoin
Core wallet (see commit d447ded6ba),
the setting doesn't have any effect and hence can be removed.

test/functional/mempool_limit.py

@@ -25,7 +25,6 @@ class MempoolLimitTest(BitcoinTestFramework):
         self.extra_args = [[
             "-datacarriersize=100000",
             "-maxmempool=5",
-            "-spendzeroconfchange=0",
         ]]
         self.supports_cli = False