6979279: remove special-case code for ParallelGCThreads==0
Reviewed-by: jwilhelm, brutisso, kbarrett
^build/
^dist/
^webrev
/nbproject/private/
^src/share/tools/hsdis/build/
^src/share/tools/IdealGraphVisualizer/[a-zA-Z0-9]*/build/
^src/share/tools/IdealGraphVisualizer/build/
^src/share/tools/IdealGraphVisualizer/dist/
^.hgtip
.DS_Store