Refactor: libcrmcommon: Remove ipc_size_max value.

With pcmk__ipc_buffer_size removed, this value no longer does anything.
It's part of the pcmk_ipc_api_s struct which does get exposed publically
as pcmk_ipc_api_t (used by sbd at least), however the members of this
struct are not public. Thus it can be removed.