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.

Signed-off-by: Venky Shankar <vshankar@redhat.com>