Changing schedule after a programme is uploaded can leave .liq files hanging around

[dnas2]

If the following occurs:

1. ProgrammeA is in the schedule
2. ProgrammeA is uploaded and the corresponding .liq file is created
3. ProgrammeA is replaced by ProgrammeB in the schedule
4. ProgrammeA's MP3 file gets deleted from the server
5. ProgrammeB is uploaded and the corresponding .liq file is created

Then there will be two .liq files for the same time. The earlier PID alphabetically has precedence, so if ProgrammeA's PID is alphabetically before ProgrammeB, we will get a silence detect as the file has been deleted.
Solution is to check in checkFilePresent.py that if there are .liqs with no corresponding .mp3, the .liq gets deleted
Worst case, if we delete too many .liqs, they'll get recreated by makeSchedule.py a couple of mins later