LINUX: Remove unused osi_fetchstore.c

LINUX: Remove unused osi_fetchstore.c

Ever since commit ae5f411c (Linux 4.4: Do not use splice()), most of
osi_fetchstore.c has been '#if 0'd out. The only portion that isn't is
a function definition that is unreferenced (afs_linux_read_actor).

Remove the unused code, and other '#if 0' references to it; the code
can always be added back later when we can actually use it.

Reviewed-on: https://gerrit.openafs.org/13061
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 96a4bee20d42484148d163b85ca049dcc980a7a5)

Change-Id: I224a630f6f3f1d9e0d08a6cd7eae3d3b871b2bd6
Reviewed-on: https://gerrit.openafs.org/13074
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>