Bug 1695580 - Remove mochitest rdd leak threshold. r=mjf,mccr8

It's unclear whether this bug fixes the leaks that prompted this leak threshold,
or whether they were already fixed. It is however clear that mochitests now run
without leaking.

Differential Revision: https://phabricator.services.mozilla.com/D109784

testing/mochitest/mochitest_options.py

@@ -1174,7 +1174,6 @@ class MochitestArguments(ArgumentContainer):
             "forkserver": options.defaultLeakThreshold,
             # GMP rarely gets a log, but when it does, it leaks a little.
             "gmplugin": 20000,
-            "rdd": 400,
         }
 
         # See the dependencies of bug 1401764.