[DOCS] envvar PGREALM seems no longer used

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/libpq-envars.html
Description:

Environment variable PGREALM seems to have been removed 12 years ago, but it
still exists in the documentation.

https://github.com/postgres/postgres/commit/a159ad30489acb7aeb35d95de805f81c64d07966