librbd: remove limit on number of objects in the cache

The number of objects is not a significant indicated of when data
should be written out for rbd. Use the highest possible value for
number of objects and just rely on the dirty data limits to trigger
flushing. When the number of objects is low, and many start being
flushed before they accumulate many requests, it hurts average request
size and performance for many concurrent sequential writes.

Fixes: #7385
Backport: emperor, dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
ObjectCacher: remove unused target/max setters

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>