tailcfg, wgengine/filter: remove most FilterRule.SrcBits code

The control plane hasn't sent it to clients in ages.

Updates tailscale/corp#20965

Change-Id: I1d71a4b6dd3f75010a05c544ee39827837c30772
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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