Remove dead code related to VerifyThread and verify_thread()

The develop flag VerifyThread:

  develop(bool, VerifyThread, false, \
          "Watch the thread register for corruption (SPARC only)")

is listed as a SPARC only flag and we no longer have a SPARC port. However, the PPC and S390 ports were based on the SPARC port and so they inherited the verify_thread() function. But that is defined as:

oid MacroAssembler::verify_thread() {
  if (VerifyThread) {
    unimplemented("", 117);
  }
}

so is not usable if VerifyThread were ever set. Consequently all this code can be removed.

The ARM port also references VerifyThread though its use is unclear:

  // setup code generation tools
  int pad = VerifyThread ? 512 : 0;
#ifdef _LP64
  CodeBuffer buffer("uncommon_trap_blob", 2700+pad, 512);
#else
  // Measured 8/7/03 at 660 in 32bit debug build (no VerifyThread)
  // Measured 8/7/03 at 1028 in 32bit debug build (VerifyThread)
  CodeBuffer buffer("uncommon_trap_blob", 2000+pad, 512);
#endif

and

 int pad = VerifyThread ? 256 : 0;// Extra slop space for more verify code

  // setup code generation tools
  // Measured 8/7/03 at 256 in 32bit debug build (no VerifyThread)
  // Measured 8/7/03 at 528 in 32bit debug build (VerifyThread)
  CodeBuffer buffer("exception_blob", 600+pad, 512);

It isn't clear what exactly was measured but in 2003 there was no ARM port and this was simply copied across from the SPARC port as well.