Merge pull request #870 from hasegeli/log_line_prefix

PostgreSQL 10 comes with "%m [%p] " [1] as the default. Deb packages are defaulted to "%t [%p-%l] %q%u@%d ", RPM packages are defaulted to "< %m >" for some time. I believe most distributions have something similar. These are all better than what we are overriding the value by default.

Removing this default would allow people to specify the value from different classes.

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=7d3235ba42f8d5fc70c58e242702cc5e2e3549a6;hp=5ff4a67f63fd6d3eb01ff9707d4674ed54a89f3b