remove convert_to_flat_in_component_mass

This has been removed from bilby_pipe. Instead, the priors can be specified:

chirp_mass = bilby.gw.prior.UniformInComponentsChirpMass(name='chirp_mass', minimum=25, maximum=35, unit='$M_{\odot}$')
mass_ratio = bilby.gw.prior.UniformInComponentsMassRatio(name='mass_ratio', minimum=0.125, maximum=1)

I think no additional post-processing code will be required in parallel-bilby's end to convert from flat-in-chirp to flat-in-component.

@gregory.ashton @colm.talbot could one of you confirm? thanks!