So unfortunately, you have discovered one of the ways that some people, in the earlier days, used to grief servers.

Summoning in a high level mob and killing causes the game to crash due to the amount of "loot" that is suddenly being deposited into the world all at once...

Using /killall is NOT the answer here... will still lag out and crash if it is killed.

I could be wrong but, I don't believe there is a way to delete it, and think you would need to rollback your server to a previous save (prior to the summoning)