mirb: use ISSPACE/ISALNUM from mruby.h, remove stale ctype.h

Remove redundant local ISSPACE/ISALNUM definitions from
mirb_completion.c and unused ctype.h includes from both
mirb_completion.c and mirb.c. The locale-independent macros
from mruby.h are already available via <mruby.h>.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bin-mirb/tools/mirb/mirb.c

@@ -24,8 +24,6 @@
 
 #include <stdlib.h>
 #include <string.h>
-#include <ctype.h>
-
 #include <signal.h>
 
 #ifdef _WIN32

mrbgems/mruby-bin-mirb/tools/mirb/mirb_completion.c

@@ -16,7 +16,6 @@
 #include <mruby/value.h>
 #include <mruby/variable.h>
 
-#include <ctype.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -51,14 +50,6 @@ strndup(const char *s, size_t n)
 #include <linenoise.h>
 #endif
 
-/* Helper macros */
-#ifndef ISSPACE
-#define ISSPACE(c) isspace((unsigned char)(c))
-#endif
-#ifndef ISALNUM
-#define ISALNUM(c) isalnum((unsigned char)(c))
-#endif
-
 /* ============================================================
  * Core Completion Engine
  * ============================================================ */