Rightholder should be able to remove a distributor

[DanielMelero]

Rightholder should have full control over the distribution of their music. Therefore if they consider that a distributor is not providing the proper service (high fees, low latency, unreachable host, etc) it should be possible to remove it from the list.

Add function:

function remove_distributor(bytes32 _song, address _dist_addr) external;

and use new modifier:

modifier onlyRightholder(bytes32 _song) {
        require(msg.sender == songs[_song].rightholder, "Only Rightholder is allowed");
        _;
}