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

require_once 'scripts/__init_script__.php';

$file = require 'conf/default.conf.php';
$global = new PhabricatorConfigDefaultSource();
$global_values = $global->getAllKeys();

foreach ($file as $key => $value) {
  $global_value = idx($global_values, $key, (object)array());

  if ($value !== $global_value) {
    echo "{$key}\n\n";
    echo "FILE VALUE\n";
    var_dump($value);
    echo "\n";
    echo "DEFAULT VALUE\n";
    var_dump($global_value);
    return;
  }
}
```

These were the keys that had issues:

  - `log.access.format` Not specified in default.conf.php, safe to speciy.
  - `mysql.pass` Empty string in file, null in global. Same effect.
  - `metamta.default-addrress` One used `noreply@example.com`, one `noreply@phabricator.example.com`. These are just human-readable examples so it's safe to change behavior.
  - `metamta.domain` same as above, `example.com` vs `phabricator.example.com`.
  - `phpmailer.smtp-host` One used null, one empty string.
  - `phpmailer.smtp-protocol` As above.
  - `files.viewable-mime-types` File version is out of date.
  - `repository.default-local-path` Null in file, set in global. This is correct to set to a default value now.
  - `pygments.dropdown-choices` File version is out of date.
  - `environment.append-paths` File version is empty, global version adds common paths. This //could// change behavior, but the web behavior is better and more reasonable in general, and a system would need to be configured in a very bizarre way for this to be relevant.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6230

Differential Revision: https://secure.phabricator.com/D10628