SyncEngine: Don't close db when done #7141

SyncEngine: Don't close db when done #7141

The db-close operation is likely a leftover from when the SyncEngine
owned its own db connection and serves no purpose anymore.

Closing the db causes the removal of the temporary wal and shm files.
These files are recreated when the db is opened again, which happens
almost immediately.

This is a problem for winvfs because the delete-recreate step wipes the
exclusion state on these files just after the sync is done. That meant
that the db temporaries permanently had a "needs sync" icon marker shown
in the explorer.

Avoiding reopening the db also reduces the number of log messages per
sync.