LU-12236 gss: remove unused code in gss_svc_upcall.c

LU-12236 gss: remove unused code in gss_svc_upcall.c

Delete rsc_flush() related functions which are never
used.

Test-Parameters: envdefinitions=SHARED_KEY=true testlist=sanity,recovery-small,sanity-sec
Change-Id: Iedd6339b5fafdea81147c83e5f0499fa3ad60251
Signed-off-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-on: https://review.whamcloud.com/34794
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>