[spu] Don't call set_gdbarch_cannot_step_breakpoint in spu_gdbarch_init

[spu] Don't call set_gdbarch_cannot_step_breakpoint in spu_gdbarch_init

Nowadays, in infrun.c:resume, the setting to 'step' variable is like:

  if (use_displaced_stepping (gdbarch)
      && tp->control.trap_expected
      && sig == GDB_SIGNAL_0
      && !current_inferior ()->waiting_for_vfork_done)
    {
    }
  /* Do we need to do it the hard way, w/temp breakpoints?  */
  else if (step)
    step = maybe_software_singlestep (gdbarch, pc); <-- [1]

  ...

  if (execution_direction != EXEC_REVERSE
      && step && breakpoint_inserted_here_p (aspace, pc))
    {
      ...
      if (gdbarch_cannot_step_breakpoint (gdbarch)) <-- [2]
        step = 0;
    }

spu doesn't have displaced stepping and uses software single step,
so 'step' is set to zero in [1], and [2] becomes unreachable as a
result.  So don't have to call set_gdbarch_cannot_step_breakpoint
in spu_gdbarch_init.

gdb:

2015-04-08  Yao Qi  <yao.qi@linaro.org>

	* spu-tdep.c (spu_gdbarch_init): Don't call
	set_gdbarch_cannot_step_breakpoint.