Reduce count of injection snr-chisq plots

[tdent]

In the multi-ifo plotting there are currently large numbers of near-identical 'injections-on-top' plots with the same injections and different coinc backgrounds, however the different backgrounds are not well visible on these and are better represented in the background-on-top plots anyway.

Hence, replace the multiple injections-on-top plots for a given ifo and colorscale with a single injections-only plot.