Please Keep Items Sets
There's no harm in keeping them even if only 0.0000001% of people use them.
It's a bonus feature with no negatives behind it. Removing it only gives negative feedback because nobody is going to go "Wow thanks riot those custom item sets were really making me trip out".
Please keep them. When I help new players I have them make an item set so they can buy the right items, otherwise we got an Ashe rushing thornmail and blood thirster. For more experienced players, it really saves time
So much simpler than going "Well it recommends that I buy BT so let me click on BT so I get can get to BF sword so I can get to IE". It shows more build options and build flexibility. I have like 15-20 items in these sets that are actually good items for any situation, recommended does not fulfill that.