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

Change-Id: I780efd55dea33f1f7893b68285c841e80660b3f3
Reviewed-on: http://gerrit.openafs.org/9608
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>