Remove $help_path

Advanced help scans both core and contrib for help files.

There is also a path known internally as $help_path constructed as follows:

$help_path = drupal_get_path('module', 'advanced_help') . '/modules';

It permits placing help files in a subdirectory named modules below the advanced_help directory.

AFAIK, this possibility has never been documented.

It does not seem useful, as any files placed there will be overwritten by new releases.

It is now removed in the latest development snapshot of the 7.x-1.x branch, and will also be removed in the next release.