Remove INVOLVEDPEOLE mappings such as PRODUCER_INVOLVED PEOPLE and simplify logic

[IJabz]

For ID3 PRODUCER used to map to IPLS with role producer, recently we modified so PRODUCER just went to a TXXX:PRODUCER field, and then created a new PRODUCER_INVOLVEDPEOPLE field that acted like the old PRODUCER field, we had similar logic for ENGINEER, MIXER and DJMIXER.

However having used the new version with SongKong I realize I am not using the PRODUCER_INVOLVEDPEOPLE fields, instead I just interact with the INVOLVEDPEOPLE field directly if I want to add a producer to the field, and also logic such as when you delete INVOLVEDPEOPLE field it doesn't delete the whole field if has one of the named roles (such as producer) was causing me problems.

Actually, simplicity is best so I am going to remove all the XXX_INVOLVEDPEOPLE logic

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[IJabz]

Done.