Removing the STGNEW and STGDELETE volhist entries does not affect any data
that may be on those tapes.
Those entries do not participate in any type of DB recovery, either.

The only thing I know of that those entries are used for is to provide a
good idea what tapes need to be audited, if you ever have to do a DB restore
and back-level your system.

I believe it is safe to delete any of those entries that are older than your
oldest DB backup.