os/bluestore: do not set osd_memory_target default from cgroup limit

os/bluestore: do not set osd_memory_target default from cgroup limit

On the aarch64 box I'm testing, this gives us a value of
7378697629483768832, which is not what we want.

I think we are better off relying on this limit being explicitly set via
environment variables (POD_* by kuberentes/rook) or via the command line.

This partially reverts 5c6b533697814af8acfd9e731a2599b2294687ef, but not
all of it, since we wan to keep the option itself, as it is now used by
common/config.cc when dealing with the POD_MEMORY_LIMIT env var.

Signed-off-by: Sage Weil <sage@redhat.com>