Double-check if we still need to handle segment1IsValid && segment2IsValid in mergeGraph

[fm3]

During #8367 we noticed that this check may be obsolete due to the agglomerates now always being looked up by segment id in the current mapping.

Maybe we can remove it? Should be double-checked.

Also, with the same justification, should we also duplicate the agglomerateGraph2.segments.contains(segmentId2) assertions for agglomerateGraph1?