net: remove an unused sync.Map in the fake net implementation

I added this map in CL 526117, but it is apparently unused.
I assume that I removed all uses of it while revising that change.

Updates golang#59718.
Updates golang#50216.

Change-Id: I8cdac39f4764d1fcc31566408304c850cf0f9374
Reviewed-on: https://go-review.googlesource.com/c/go/+/557176
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/net/net_fake.go

@@ -23,7 +23,6 @@ import (
 
 var (
 	sockets         sync.Map // fakeSockAddr → *netFD
-	fakeSocketIDs   sync.Map // fakeNetFD.id → *netFD
 	fakePorts       sync.Map // int (port #) → *netFD
 	nextPortCounter atomic.Int32
 )