memory.h was introduced back when the shared_ptrs were still in TR1, but
we've moved to C++17 now. and the C++ clients should be compiled with
a C++11 compatible compiler. so there is no need to have this file anymore.
also replace all references of ceph::shared_ptr and ceph::weak_ptr with
std::shared_ptr and std::weak_ptr accordingly.
Signed-off-by: Kefu Chai <kchai@redhat.com>