logging: Remove syscall for z_log_msg_runtime_vcreate

This syscall is completely problematic in userspace, it does
not check ANY parameter that is given and it uses variadic argument
that are not copied / checked before being used in the implementation,
instead it just pass a pointer to user stack with unknown data is
blindly consumed by the kernel.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>