Use disassemble.c:disassembler select rs6000 disassembler

Use disassemble.c:disassembler select rs6000 disassembler

Nowadays, rs6000 disassembler is selected in different ways in
opcodes and gdb,

opcodes:
    case bfd_arch_rs6000:
      if (mach == bfd_mach_ppc_620)
	disassemble = print_insn_big_powerpc;
      else
	disassemble = print_insn_rs6000;
      break;

gdb:

  if (arch == bfd_arch_rs6000)
    set_gdbarch_print_insn (gdbarch, print_insn_rs6000);
  else
    set_gdbarch_print_insn (gdbarch, gdb_print_insn_powerpc);

I am not sure which one is the right one.  However, such selection
should be done in one place instead of two.

gdb:

2017-05-24  Yao Qi  <yao.qi@linaro.org>

	* rs6000-tdep.c (gdb_print_insn_powerpc): Remove.
	(rs6000_gdbarch_init): Don't call set_gdbarch_print_insn.