John Gill Commentary Exodus 8:31

"And Jehovah did according to the word of Moses; and he removed the swarms of flies from Pharaoh, from his servants, and from his people; there remained not one." — Exodus 8:31 (ASV)

And the Lord did according to the word of Moses
Did as he entreated him to do, as follows: and he removed the swarms of flies from Pharaoh, from his servants, and
from his people ;
by what means is not said, whether by destroying them at once, as the frogs, or by driving them away with a wind, as the locusts afterwards: there remained not one ;
the meaning is not, not one swarm of flies, but not one fly, there was not one left; which looks as if it was in the latter way that they were removed, since, if in the former, they would have remained, though dead, as the frogs did, for a little while.