remove backtrace functions

remove backtrace functions

* src/eval.c (backtrace_function, backtrace_nargs, backtrace_args)
  (backtrace_debug_on_exit, set_backtrace_args)
  (set_backtrace_debug_on_exit, backtrace_p, backtrace_top)
  (backtrace_next, do_debug_on_call, record_in_backtrace)
  (Fbacktrace_debug, Fbacktrace, get_backtrace_frame)
  (Fbacktrace_frame, backtrace_eval_unrewind, Fbacktrace_eval)
  (get_backtrace, backtrace_top_function, Vsignaling_function):
  Remove. All references changed.

* src/lisp.h (SPECPDL_BACKTRACE): Remove. All references changed.