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

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 #59718.
Updates #50216.

Change-Id: I8cdac39f4764d1fcc31566408304c850cf0f9374
Reviewed-on: https://go-review.googlesource.com/c/go/+/557176
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>