Remove SyncHistory.json

[philosowaffle]

Historically, P2G has written workout information into the SyncHistory.json file. P2G used the information in this file to keep track of which workouts it had already downloaded from Peloton, so it could skip re-downloading them again. Originally, this also served the purpose of avoiding duplicate uploads to Garmin.

However, this file is becoming problematic:

1. It is tricky to maintain and use in the code
2. It causes a lot of confusion for users
3. It makes it difficult to re-sync a workout that might have failed because the user must know to go in and delete the file
4. In reality, it does not improve the performance of P2G, and is not needed to avoid duplicates on Garmin

For this reason, I am planning to stop writing the SyncHistory.json file. This change should be passive and should not impact anyone's day to day usage of P2G, on the contrary it should make it easier to re-sync failed workouts in the future. This work also pairs well with #26 .