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.

This patch is back-ported from the following one:
Lustre-commit: 25b0bf5a23032394055b7b94b3169f5cf4068570
Lustre-change: https://review.whamcloud.com/34794

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/38476
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>