Do not remove LSIF upload files immediately on deletion.

Created by: efritz

This simplifies the deletion path for upload files. We have a task that already removes files that are no longer referenced by an LSIF upload row in the database. The current code is a bit sketchy when it's part of a larger transaction that rollsback - it's possible to get into a state where we remove the file but not the row, causing reference issues later