Stop coherence plots of combined sky maps

Currently we are still plotting the coherence combined sky maps. However, these don't change this metadata and so will always be redundant compared to their GW sky map counterpart and we've have code that is supposed to block this but still has a bug: https://git.ligo.org/emfollow/gwcelery/-/blob/bf057cd88dbec2c13c73e71b452404bb39bf11a0/gwcelery/tasks/skymaps.py#L338-341

This could be fixed by changing this and with an or to ignore all combined sky maps regardless of whether they end with fits (which they all currently do).