ipn/ipnlocal: remove another copy of slicesx.MapValues

We added this helper in 1e2e319e7d. Remove this copy.

Updates #cleanup

Change-Id: I5b0681acc23692beed35951c9902ac9ceca0a8b9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f