R1:b16527d93dce

Remove old, confusing configuration files

Summary:
Fixes T6230. These files have not been read by default for a long time, but users are frequently confused and try to edit `default.conf.php`.

Remove the actual files. Allow `phabricator_read_config_file(...)` to continue working as though they exist so as to not break config-file-based installs.

Test Plan:
I used this script to make sure that removing `default.conf.php` won't change things for installs which are still using config files:

```
<?php…