Revert "Add a new function `mrb_exc_protect()`."

This reverts commit 8746a6fe4e7bda8a0fbc0eaece9314ec51a0c255.

We already have `mrb_protect()`, `mrb_ensure()` and `mrb_rescue()`
functions. If you need to handle exceptions from C functions, use those
functions above.

include/mruby.h

@@ -1293,9 +1293,6 @@ MRB_API void mrb_print_backtrace(mrb_state *mrb);
 MRB_API void mrb_print_error(mrb_state *mrb);
 /* function for `raisef` formatting */
 MRB_API mrb_value mrb_vformat(mrb_state *mrb, const char *format, va_list ap);
-/* function to protect errors during execution */
-MRB_API mrb_value mrb_exc_protect(mrb_state *mrb, mrb_value (*body)(mrb_state*, void*), void *a, mrb_value (*resc)(mrb_state*, void*, mrb_value), void *b);
-
 
 /* macros to get typical exception objects
    note:

src/error.c

@@ -555,29 +555,6 @@ mrb_argnum_error(mrb_state *mrb, mrb_int argc, int min, int max)
 #undef FMT
 }
 
-MRB_API mrb_value
-mrb_exc_protect(mrb_state *mrb, mrb_value (*body)(mrb_state*, void*), void *a,
-                mrb_value (*resc)(mrb_state*, void*, mrb_value), void *b)
-{
-  struct mrb_jmpbuf *prev_jmp = mrb->jmp;
-  struct mrb_jmpbuf c_jmp;
-  mrb_value v = mrb_undef_value();
-
-  MRB_TRY(&c_jmp) {
-    mrb->jmp = &c_jmp;
-    v = body(mrb, a);
-  } MRB_CATCH(&c_jmp) {
-    if (mrb->exc) {
-      v = resc(mrb, b, mrb_obj_value(mrb->exc));
-      mrb->exc = NULL;
-    }
-  } MRB_END_EXC(&c_jmp);
-
-  mrb->jmp = prev_jmp;
-
-  return v;
-}
-
 void mrb_core_init_printabort(void);
 
 int