Fix unused warnings

http://ci.rvm.jp/results/trunk_gcc7@silicon-docker/2539622
```
/tmp/ruby/v2/src/trunk_gcc7/class.c: In function 'rb_scan_args_parse':
/tmp/ruby/v2/src/trunk_gcc7/class.c:1971:12: warning: unused variable 'tmp_buffer' [-Wunused-variable]
     VALUE *tmp_buffer = arg->tmp_buffer;
            ^~~~~~~~~~
```
```
In file included from /tmp/ruby/v2/src/trunk_gcc7/vm_insnhelper.c:1895:0,
                 from /tmp/ruby/v2/src/trunk_gcc7/vm.c:349:
/tmp/ruby/v2/src/trunk_gcc7/vm_args.c:212:1: warning: 'args_stored_kw_argv_to_hash' defined but not used [-Wunused-function]
 args_stored_kw_argv_to_hash(struct args_info *args)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
```

1 files changed, 0 insertions, 1 deletions

diff --git a/class.c b/class.c
index e42ee0821c..7db54d6038 100644
--- a/class.c
+++ b/class.c
@@ -1968,7 +1968,6 @@ static void
 rb_scan_args_parse(int kw_flag, int argc, const VALUE *argv, const char *fmt, struct rb_scan_args_t *arg)
 {
     const char *p = fmt;
-    VALUE *tmp_buffer = arg->tmp_buffer;
     int keyword_given = 0;
     int last_hash_keyword = 0;