Remove hack forcing MPEG1 video to be treated as MPEG2

This hack was added to fix #287, which was a problem with XvMC decoding.
It makes no sense today, and actually break genuine MPEG1 videos.

Fixes #12160
Don't attempt to issue DB query if there's nothing to do

Prevent harmless SQL statement error messages to be logged when the position map is empty

Regression introduced in b4be8e24f9fab5b6a71f62df8e2c0403bedf4805

Fixes #12161