baseparse: Don't truncate the duration to milliseconds in gst_base_parse_convert_default().

There's no need to do this, and it can make seeking far less accurate.

For a specific use case: I am working with a long (45-minute) MPEG-1 layer 3 file, which has a constant bit rate but no seeking tables. Trying to seek the pipeline immediately after pausing it, without the ACCURATE flag, to a location 41 minutes in, yields a location that is potentially over ten seconds ahead of where it should be. This patch improves that drastically.