ipc4: mixin/mixout: Remove redundant source_info stuff

Mutex protected module_source_info was an old failed attempt to support
cross-core connection of mixin and mixout. That functionality was never
properly tested as FW did not support cross-core connections there.

For connected mixin and mixout located on same core, that functionality
is not needed. For cross-core connection, that functionality creates
dead-lock.

Signed-off-by: Serhiy Katsyuba <serhiy.katsyuba@intel.com>