remove checkpoint_warning

the checkpoint_warning feature was added by commit 2986aa6a668bce3cfb836
in November 2002 when we didn't have any logging of checkpointing at
all.  I propose to remove it: surely anyone who cares about analyzing
checkpointing behavior nowadays is using the log_checkpoint feature
instead, which contains much more detail.  The other one is just noise
now, and probably ignored amidst the number of other warning traffic.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services