WebStart: Remove use of realpath() for $IP

WebStart: Remove use of realpath() for $IP

When installing MediaWiki in a sub directory of document root,
and including it from an /index.php file in the document root,
MediaWiki succesfully includes WebStart from index.php, but
WebStart.php fails to include Setup.php.

For example, MediaWiki installation at /var/www/mediawiki with the
following file at /var/www/index.php.

```
<?php
require __DIR__ . '/mediawiki/index.php';
```

Failure:

> Fatal error:
> require_once(): Failed opening required '/var/www/includes/Setup.php'
> (include_path='.:/usr/local/lib/php') in
> /var/www/mediawiki/includes/WebStart.php on line 97
>
> Stack trace:
>   1. {main}() /var/www/index.php:0
>   2. require() /var/www/index.php:3
>   3. require() /var/www/mediawiki/index.php:40

Bug: T153882
Change-Id: Icd8cfa580ce1c22bc3bf177570a9f4a940d2427c