Cleanup for log_min_messages changes in 38e0190ced71

Cleanup for log_min_messages changes in 38e0190ced71

* Remove an unused variable
* Use "default log level" consistently (instead of "generic")
* Keep the process types in alphabetical order (missed one place in the
  SGML docs)
* Since log_min_messages type was changed from enum to string, it
  is a good idea to add single quotes when printing it out.  Otherwise
  it fails if the user copies and pastes from the SHOW output to SET,
  except in the simplest case.  Using single quotes reduces confusion.
* Use lowercase string for the burned-in default value, to keep the same
  output as previous versions.

Author: Euler Taveira <euler@eulerto.com>
Author: Man Zeng <zengman@halodbtech.com>
Author: Noriyoshi Shinoda <noriyoshi.shinoda@hpe.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/202602091250.genyflm2d5dw@alvherre.pgsql