Remove debug flag from test runs

Profiling of test262parallel shows that removing -Dnashorn.debug=true system property from test runs is expected to cut down the runtime of test262parallel by ~10%, as it doesn't have to use debugger call sites that track per-call-site link-miss counters. If needed, they can be reenabled on a per-execution basis with

"ant -Dnashorn.debug=true test262parallel"