Remove num_xloginsert_locks GUC, replace with a #define

Remove num_xloginsert_locks GUC, replace with a #define

I left the GUC in place for the beta period, so that people could experiment
with different values. No-one's come up with any data that a different value
would be better under some circumstances, so rather than try to document to
users what the GUC, let's just hard-code the current value, 8.