remove internal treatment of default prompt tag, it seems there was no need

remove internal treatment of default prompt tag, it seems there was no need

* libguile/control.h (scm_sys_default_prompt_tag):
* libguile/control.c (scm_init_control): Remove the logic that defined
  %default-prompt-tag.
  (scm_c_abort): Remove check for default prompt tag, it wasn't useful.

* libguile/throw.c (sym_pre_init_catch_tag): Define as the pre-init
  prompt tag.
  (pre_init_catch, pre_init_throw): Use sym_pre_init_catch_tag.

* module/ice-9/boot-9.scm (default-prompt-tag): Define as a simple
  value, not a fluid. Perhaps we can expose it as a fluid later.