Remove TODO comment and deprecate method

Issues from TODOs

Line 370 of file ObsService.java
Link: https://github.com/openmrs/openmrs-core/blob/master/api/src/main/java/org/openmrs/api/ObsService.java#L370
contains TODO

Get a complex observation. If obs.isComplex() is true, then returns an Obs with its ComplexData. Otherwise returns a simple Obs.
Possibly remove this method. It is confusing because you may have a complexObs, but not want the complexData attached at the time. Nevertheless, you may think that this method is necessary to use, when you could just call getObs(). This will attach the complexData onto the obs