net/tsaddr: remove ContainsFunc helpers (they're now in x/exp/slices)

x/exp/slices now has ContainsFunc (golang/go#53983) so we can delete
our versions.

Change-Id: I5157a403bfc1b30e243bf31c8b611da25e995078
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f