mruby.h: remove mrb_str_format declaration

The function mrb_str_format is only defined when the mrbgem mruby-sprintf was
activated. That might not always be the case (by default it currently is).
mrb_str_format still has external linkage, so from now on mrbgem authors
have to declare it in their source code if they want or need to call it.

include/mruby.h

@@ -240,7 +240,6 @@ mrb_value mrb_check_intern_str(mrb_state*,mrb_value);
 const char *mrb_sym2name(mrb_state*,mrb_sym);
 const char *mrb_sym2name_len(mrb_state*,mrb_sym,size_t*);
 mrb_value mrb_sym2str(mrb_state*,mrb_sym);
-mrb_value mrb_str_format(mrb_state *, int, const mrb_value *, mrb_value);
 
 void *mrb_malloc(mrb_state*, size_t);         /* raise RuntimeError if no mem */
 void *mrb_calloc(mrb_state*, size_t, size_t); /* ditto */