mtr: Fix rpl_parallel_multi_db for ASAN

Summary:
This test was overriding slave opts to use a 512K thread stack, which
was once upon a time an increase from the 280K default, but has long
since been a decrease. Since the [new default is 10MB](5a1fc32) in ASAN (and [1MB otherwise](907e8ab)),
we can just remove the override and allow the default to take effect.

Differential Revision: D51241358

fbshipit-source-id: b102f2400a1048d96874e5eeaf1e13a7487e74b2

mysql-test/suite/rpl/t/rpl_parallel_multi_db-slave.opt

@@ -1,2 +1 @@
---thread_stack=512K --replica-transaction-retries=0 --log-error-verbosity=1
-
+--replica-transaction-retries=0 --log-error-verbosity=1