Remove log_checkpoints = true from .pl tests

Hi,

With commit 64da07c41a8c0a680460cdafc79093736332b6cf making default
value of log_checkpoints to on, do we need to remove explicit settings
in perl tests to save some (5) LOC?

Although, it's harmless, here's a tiny patch to remove them.

--
Bharath Rupireddy
RDS Open Source Databases: https://aws.amazon.com/rds/postgresql/