Remove unnecessary methods following workouts updates

[SamR1]

Next part of file processing rewriting (#925):

• remove unnecessary storage of generated GPX file when the file extension is not .gpx (and add GPX generation on file download). The original file is still stored to allow data refresh.
• use uuid instead of WorkoutSegment composite primary key (workout_id, segment_id)
• remove methods based on gpx file to get chart data and to display map