fix(config): path is derived from PHP, not database

Elgg falls apart if `$CONFIG->path` is set to anything but the Elgg root,
so there is no point in saving a different value to the DB
or in respecting any value already saved to the DB.

Refs #2316
Merge pull request #7069 from ewinslow/config-b2316

fix(config): path is derived from PHP, not database