Delete absolute filename ImportedHitsFiles

Each actual hits file will have many records in this table because the
filenames were being stored as absolute paths, including a timestamped
directory per deploy. This cleans those old records up so that we will
have only one row per real hits file with a relative path filename.

This will delete only rows with an absolute filename, in case someone
wants to rollback in dev and then migrate forwards through this again
without losing any relative-filename tracking.