stringio.c: unused variable

* ext/stringio/stringio.c (strio_memsize): remove a variable which
  is no longer used.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52991 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/stringio/stringio.c

@@ -66,7 +66,6 @@ strio_free(void *p)
 static size_t
 strio_memsize(const void *p)
 {
-    const struct StringIO *ptr = p;
     return sizeof(struct StringIO);
 }