BUGFIX: Search reindex task clearing search index for subsites.

Fixes silverstripe-labs/silverstripe-fulltextsearch#26.

Regression issue caused by this change in subsites:
https://github.com/silverstripe/silverstripe-subsites/commit/7bf6e893208ce689b45ab34a9c1de14e5d5201bf

When reindexing the subsites the session subsite ID should be used by the reindex task.