For me it works with igraph0.6:
import random
import igraph
N = 10
g = igraph.Graph.Full(N)
i, j = random.sample(range(g.vcount()), 2)
g.delete_edges([(i,j)])
print(g.ecount(), N*(N-1)/2)
From the output can be concluded, it has deleted one edge:
44 45.0