Fix memory-leak in SessionFactoryImpl::sessionClosed.

[vnayar]

The std.algorithm.remove() method does not modify in-place an array, which causes 'activeSessions' to accumulate entries and never remove them.

See https://dlang.org/library/std/algorithm/mutation/remove.html

Note that remove does not change the length of the original range directly; instead, it returns the shortened range. If its return value is not assigned to the original range, the original range will retain its original length, though its contents will have changed:

int[] a = [ 3, 5, 7, 8 ]; assert(remove(a, 1) == [ 3, 7, 8 ]); assert(a == [ 3, 7, 8, 8 ]);

[SingingBush]

sorry I forgot to merge this, it seems fine. Would you be able to add a quick unit test though

[vnayar]

sorry I forgot to merge this, it seems fine. Would you be able to add a quick unit test though

I couldn't quite figure out a way of making a true unit-test, due to the explicit dependency on SessionImpl, but I added a small entry to integration-tests instead.