Fix clang warning introduced with !1553
After !1553 (merged), I got these warning from clang16
../GaudiHive/src/CPUCruncher.cpp:161:39: warning: unused variable 'startSleeptbb' [-Wunused-variable]
tbb::tick_count startSleeptbb;
^
../GaudiHive/src/CPUCruncher.cpp:162:39: warning: unused variable 'endSleeptbb' [-Wunused-variable]
tbb::tick_count endSleeptbb;
^