Remove thread priority tweaking

Back in 2009 when this was added it improved scheduling of lookahead threads on prevalent operating systems at the time.

According to more recent testing by Intel however, lowering thread priorities does not improve performance on modern operating systems. And more importantly, doing so on systems with heterogeneous CPU topologies may actually result in a severe performance reduction.

Removing this code altogether eliminates the issue with performance degradation on such systems, while having no noticeable impact on regular systems with homogeneous CPU topologies.