API: libcrmcluster: Drop enum crm_ais_msg_class and its value

Don't bother replacing it internally, since only one value has existed
since 2.0.0 and it's equal to 0. calloc() takes care of zeroing the
value in newly allocated messages.

However, the id member of pcmk__cpg_msg_t must remain, to avoid breaking
communication with older nodes during a rolling upgrade.

This breaks API backward compatibility.

Ref T793

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>