client: remove redundant abort logic during dentry invalidation test

client: remove redundant abort logic during dentry invalidation test

commit d1471f070c added retry logic when remounting is
used to invalidate kernel dcache. ceph-fuse performs a
dcache invalidation test during startup, thereby logging
(and continuing) or aborting as per certain configuration
settings. the abort logic is pretty much redundant and
can be cleaned up preserving the existing behavior.

Also, assert is none of dcache invalidation mecahnisms
(`dentry_invalidate_cb` or `remount_cb`) are available.

Fixes: http://tracker.ceph.com/issues/35932
Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit 232d12f03612983336a2e90cb024deb79e5e6934)

 Conflicts:
	src/client/Client.cc