Remove unused sun.misc.Unsafe prefetch intrinsic support

JDK-8068975 will clean up the native implementation of sun.misc.Unsafe which includes the removal of the no longer utilized prefetch read/write native methods.

The corresponding prefetch intrinsics (declared in vmSymbols.hpp) are as follows:

  do_intrinsic(_prefetchRead, sun_misc_Unsafe, prefetchRead_name, prefetch_signature, F_RN) \
   do_name( prefetchRead_name, "prefetchRead") \
  do_intrinsic(_prefetchWrite, sun_misc_Unsafe, prefetchWrite_name, prefetch_signature, F_RN) \
   do_name( prefetchWrite_name, "prefetchWrite") \
  do_intrinsic(_prefetchReadStatic, sun_misc_Unsafe, prefetchReadStatic_name, prefetch_signature, F_SN) \
   do_name( prefetchReadStatic_name, "prefetchReadStatic") \
  do_intrinsic(_prefetchWriteStatic, sun_misc_Unsafe, prefetchWriteStatic_name, prefetch_signature, F_SN) \
   do_name( prefetchWriteStatic_name, "prefetchWriteStatic") \

These and relevant code can be removed.

Note read/write prefetch support was implemented as an experiment to see if JDK library code could use it for performance advantages. However, the results of the experiment did not indicate this was worthwhile. As a consequence there are no corresponding prefetch native method declarations in sun.misc.Unsafe.