vol: remove duplicate stmp declaration

Patchset 38cf31463e3f3c675de727c1e793e117a90e6d20 added a definition of
afs_ino_str_t stmp which should have replaced the b64_string_t stmp
declaration that was already present.

Thanks to Jason Madar for noticing.

FIXES 131620

Reviewed-on: http://gerrit.openafs.org/9608
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit dd903ce729c24c9d1ed78b227a261b2549482461)

Change-Id: Ia0a43f64183b47b6df30495673907d176d95799a
Reviewed-on: http://gerrit.openafs.org/9771
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>