<prompt> has no pre-unwind-handler, it's unnecessary

<prompt> has no pre-unwind-handler, it's unnecessary

* libguile/control.h:
* libguile/control.c (scm_c_make_prompt, SCM_PROMPT_PRE_UNWIND_HANDLER):
* libguile/vm-i-system.c (prompt)
* module/language/tree-il.scm (<prompt> prompt-pre-unwind-handler):
* module/language/tree-il/analyze.scm:
* module/language/tree-il/compile-glil.scm:
* module/language/tree-il/inline.scm:
* module/language/tree-il/primitives.scm: Remove the "pre-unwind"
  handler from prompt; it turns out not to be necessary. Adapt all
  references.