Remove unnecessary code in start() method

Inside the LatencyTester test helper, we already call updateEnabled()
inside the constructor and also have a DeviceConfig property change
listener that also calls it, so we don't need to call it inside
start().

Bug: 286973787
Test: atp:v2/android-crystalball-eng/health/microbench/systemui/systemui-latency-suite
Change-Id: I4d05788771835ff51c098d11da639ede1d4ef400