Removed unused variable

`generic_iv_tbl_compat` has not been utilized since 14d61a9.

variable.c

@@ -44,7 +44,6 @@ static void check_before_mod_set(VALUE, ID, VALUE, const char *);
 static void setup_const_entry(rb_const_entry_t *, VALUE, VALUE, rb_const_flag_t);
 static VALUE rb_const_search(VALUE klass, ID id, int exclude, int recurse, int visibility);
 static st_table *generic_iv_tbl;
-static st_table *generic_iv_tbl_compat;
 
 struct ivar_update {
     union {
@@ -1007,13 +1006,6 @@ rb_free_generic_ivar(VALUE obj)
 
     if (st_delete(generic_iv_tbl, &key, (st_data_t *)&ivtbl))
 	xfree(ivtbl);
-
-    if (generic_iv_tbl_compat) {
-	st_table *tbl;
-
-	if (st_delete(generic_iv_tbl_compat, &key, (st_data_t *)&tbl))
-	    st_free_table(tbl);
-    }
 }
 
 RUBY_FUNC_EXPORTED size_t