Remove unnecessary nulling out of suites

[thatmarvin]

When a watcher triggers Karma to re-run a test, onRunComplete() from
the first test run happens after onRunStart() from the second test run.
This results in suites incorrectly clobbered to null.

Nulling it out is unnecessary as long as onRunStart() initializes
suites.

Fixes #99.

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[thatmarvin]

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